you didn't mention Amazons document conversion service

Amazon offers two forms of a conversion service, you can e-mail a document to one address and they will convert it and e-mail it back to you (for free), or you can e-mail it to a second address and tehy will convert it and load it wirelessly onto the kindle (at a charge, $0.10 or $0.15 per meg, I use the service frequently, but haven't bothered to check the exact pricing ;-)

the mobi format that the kindle accepts is used by several other book readers (it's an extention of the .prc palm format, so items in that format also work), and so far I have not found any online bookstore that does not support it, including the ones that I have checked that are supposedly tied to competitors devices.