I found a few research studies on the topic:

<1> Gooren LJ. Giltay EJ. Bunck MC. "Long-term treatment of transsexuals with cross-sex hormones: extensive personal experience." Journal of Clinical Endocrinology & Metabolism. 93(1):19-25, 2008 Jan.
Abstract
CONTEXT: Transsexuals receive cross-sex hormone treatment. Its short-term use appears reasonably safe. Little is known about its long-term use. This report offers some perspectives. SETTING: The setting was a university hospital serving as the national referral center for The Netherlands (16 million people). PATIENTS: From the start of the gender clinic in 1975 up to 2006, 2236 male-to-female and 876 female-to-male transsexuals have received cross-sex hormone treatment. In principle, subjects are followed up lifelong. INTERVENTIONS: Male-to-female transsexuals receive treatment with the antiandrogen cyproterone acetate 100 mg/d plus estrogens (previously 100 microg ethinyl estradiol, now 2-4 mg oral estradiol valerate/d or 100 microg transdermal estradiol/d). Female-to-male transsexuals receive parenteral testosterone esters 250 mg/2 wk. After 18-36 months, surgical sex reassignment including gonadectomy follows, inducing a profound hypogonadal state. MAIN OUTCOME MEASURES: Outcome measures included morbidity and mortality data and data assessing risks of osteoporosis and cardiovascular disease. RESULTS: Mortality was not higher than in a comparison group. Regarding morbidity, with ethinyl estradiol, there was a 6-8% incidence of venous thrombosis, which is no longer the case with use of other types of estrogens. Continuous use of cross-sex hormones is required to prevent osteoporosis. Androgen deprivation plus an estrogen milieu in male-to-female transsexuals has a larger deleterious effect on cardiovascular risk factors than inducing an androgenic milieu in female-to-male transsexuals, but there is so far no elevated cardiovascular morbidity/mortality. Low numbers of endocrine-related cancers have been observed in male-to-female transsexuals. CONCLUSIONS: Cross-sex hormone treatment of transsexuals seems acceptably safe over the short and medium term, but solid clinical data are lacking. [References: 40]

<2> Mueller A. Dittrich R. Binder H. Kuehnel W. Maltaris T. Hoffmann I. Beckmann MW. "High dose estrogen treatment increases bone mineral density in male-to-female transsexuals receiving gonadotropin-releasing hormone agonist in the absence of testosterone." European Journal of Endocrinology. 153(1):107-13, 2005 Jul.
Abstract
OBJECTIVE: To study the effect of estrogen (E) on the male skeleton in the absence of testosterone (T). DESIGN: Retrospective analyses of 40 middle-aged transsexuals treated with subcutaneous injections of gonadotropin-releasing hormone agonist every 4 weeks and oral 17-beta-estradiol-valerat 6 mg/day over two years until reassignment surgery. METHODS: The bone mineral density (BMD) in the femoral neck and lumbar spine (L2-L4) was measured with dual-energy X-ray absorptiometry at the beginning of cross-sex hormone treatment, after 12 and 24 months, and serum T, E, sex hormone-binding globulin (SHBG), calcitonin (CAL), osteocalcin (OSC), and urinary free deoxypyridinoline (DPD) were measured. RESULTS: After 12 months, a significant increase in BMD in the lumbar spine from 1.2 to 1.234 g/cm2 and after 24 months to 1.274 g/cm2 was observed. There was a significant increase in BMD in the femoral neck area from 1.068 to 1.109 g/cm(2) after 24 months. There was a significant decrease in serum T levels from 18.65 to 0.57 nmol/l after 12 months, and to 0.62 nmol/l after 24 months, a significant increase in SHBG levels from 50.09 to 125 nmol/l after 12 months, and to 130 nmol/l after 24 months, and a significant increase in serum E levels from 73.42 to 881.6 pmol/l after 12 months, and to 923.62 pmol/l after 24 months of cross-sex hormone treatment. Serum levels of CAL, OSC and urinary DPD were unchanged. CONCLUSION: We conclude that high dose E treatment is able to increase BMD significantly in the femoral neck and lumbar spine independently of serum T levels in middle-aged men. There is no risk of osteoporosis developing in male-to-female transsexuals receiving GnRHa when there is an adequate E substitution.

<3>
Ruetsche AG. Kneubuehl R. Birkhaeuser MH. Lippuner K. "Cortical and trabecular bone mineral density in transsexuals after long-term cross-sex hormonal treatment: a cross-sectional study." Osteoporosis International. 16(7):791-8, 2005 Jul.
Abstract
The aim of this study was to explore the effect of long-term cross-sex hormonal treatment on cortical and trabecular bone mineral density and main biochemical parameters of bone metabolism in transsexuals. Twenty-four male-to-female (M-F) transsexuals and 15 female-to-male (F-M) transsexuals treated with either an antiandrogen in combination with an estrogen or parenteral testosterone were included in this cross-sectional study. BMD was measured by DXA at distal tibial diaphysis (TDIA) and epiphysis (TEPI), lumbar spine (LS), total hip (HIP) and subregions, and whole body (WB) and Z-scores determined for both the genetic and the phenotypic gender. Biochemical parameters of bone turnover, insulin-like growth factor-1 (IGF-1) and sex hormone levels were measured in all patients. M-F transsexuals were significantly older, taller and heavier than F-M transsexuals. They were treated by cross-sex hormones during a median of 12.5 years before inclusion. As compared with female age-matched controls, they showed a significantly higher median Z-score at TDIA and WB (1.7+/-1.0 and 1.8+/-1.1, P < 0.01) only. Based on the WHO definition, five (who did not comply with cross-sex hormone therapy) had osteoporosis. F-M transsexuals were treated by cross-sex hormones during a median of 7.6 years. They had significantly higher median Z-scores at TEPI, TDIA and WB compared with female age-matched controls (+0.9+/-0.2 SD, +1.0+/-0.4 SD and +1.4+/-0.3 SD, respectively, P < 0.0001 for all) and reached normal male levels except at TEPI. They had significantly higher testosterone and IGF-1 levels (p < 0.001) than M-F transsexuals. We conclude that in M-F transsexuals, BMD is preserved over a median of 12.5 years under antiandrogen and estrogen combination therapy, while in F-M transsexuals BMD is preserved or, at sites rich in cortical bone, is increased to normal male levels under a median of 7.6 years of androgen treatment in this cross sectional study. IGF-1 could play a role in the mediation of the effect of androgens on bone in F-M transsexuals.

As always, check with a medical health professional for interpretation of these results and how they may apply to you.