Mary Helen Miller, in an article in the Chronicle of Higher Education (February 25, 2010), reports that "the president of the Pennsylvania college, Stephen G. Emerson, said this week that when he found out the letter had been stolen—from Paris's Institut de France about 170 years ago—he knew it must be returned. So in June, Mr. Emerson will fly to France with the letter in his carry-on bag, and give it back."
I'm not sure the letter provides any substantial new information on Descartes' theories, but it does appear to shed some light on the structure and composition of his Meditations on First Philosophy, indicating that he made some significant rearrangements and extractions.
[Hat tip to N.B.]