I imagine that Amazon.com's purchase of Mobipocket.com might show where they're headed, format-wise.
And I noticed that Sony's eReader
website is pretty coy about the format stuff -- the blurb reads something like "displays PDFs, personal documents, blogs, newsfeeds, and JPGs" with the small note at the bottom that they have to be converted to BBeB... which means to me that the device only reads BBeB, but they'll give you some software (Windows only, most likely) to convert a couple of different formats into BBeB. Bleah.
I much prefer the iLiad's native support for a couple of truly
open formats like HTML, and PDF.