This question was answered in the discussion list at a UK site "The Phrase Finder" (www.phrases.org.uk) in 2001, and coincidentally was reprised just 2 weeks ago:
Posted by ESC on May 15, 2005 at 19:58:17:
In Reply to:... What is the meaning and origin of "You can't have your cake and eat it too"?
From the archives:
"You can't have your cake and eat it too -- One can't use something up and still have it to enjoy. This proverb was recorded in the book of proverbs by John Heywood in 1546, and is first attested in the United States in the 1742 'Colonial Records of Georgia' in 'Original Papers, 1735-1752.' The adage is found in varying forms: You can't eat your cake and have it too. You can't have everything and eat it too; Eat your cake and have the crumbs in bed with you, etc. ..." From "Random House Dictionary of Popular Proverbs and Sayings" (1996) by Gregory Y. Titelman (Random House, New York, 1996).
Bartlett's attributes the first print record to John Heywood:
AUTHOR:John Heywood (1497?1580?)
QUOTATION:Would yee both eat your cake and have your cake?
ATTRIBUTION:Proverbes. Part ii. Chap. ix.
also noting English Metaphysical poet George Herbert's variation:
AUTHOR:George Herbert (15931633)
QUOTATION:Wouldst thou both eat thy cake and have it?
(Bartlett's Familiar Quotations
Revised and Enlarged by Nathan Haskell Dole
Boston, Little Brown, 1919
New York: Bartleby.com, 2000
The fact that Heywood placed it in a compilation of proverbs implies that it was a common saying before the mid-1500s.
The point of the saying, as I understand it, is that if you eat the cake, you no longer have it. The point of the cake is the eating, so the message seems to be about choosing carefully how to use finite resources.
again, from the "Phrase Finder" site's discussion archives:
Posted by Scott Marsden on August 17, 2000
In Reply to: "You can't have your cake and eat it, too" posted by Nostromo on August 17, 2000
: I have found where this originates from (In fact, it was the other way around: You can't eat your cake and have it, too); but does anyone know the definitive answer as to the exact meaning, or intent, of the expression?
When given a choice between 2 mutually exclusive desireable things, you can't have them both. For instance, you can either eat a cake, or you can keep it for later, but you can't do both. Unless you only eat part of the cake, but that doesn't count.. ;)