One of the publishers that had a booth at the national AMS / MAA meeting in SF this year was demonstrating math books on the Kindle. They said that the new version of the kindle software supports math fonts. I saw a demo (and entered a raffle for a free kindle) and it looks very good. No free kindle though, drat.
I looked at a sample of “Proofs from the Book” on a kindle and it was horrible. It looked as if they had tried to OCR it and it stumbled badly on the formula. This was in Dec 09. I think the Kindle is easily capable of doing the display, if the source is acceptable. Unfortunately, I doubt you will be able to check before you buy the book.
Math on the kindle varies wildly. I’ve heard awful things about some books, but I’ve got Lazarsfeld’s Positivity II and Eisenbud’s Geometry of Syzygies and both are quite well set up and easy to read, as is Silverman’s Arithmetic of Elliptic Curves.
And you CAN check, because you can always download the first bit of a book for free, and I’d recommend not buying math books unless you’ve done so.