I sent an eMail to Fictionwise yesterday noting that I wanted Sony Reader support, and that Baen's .rtf files work just fine. I suggested that they consider that route (or .pdf)for their multi-format eBooks.
They responded last night that they are actively considering how to support the Sony Reader. They didn't say which approach they will take, but did say that they expected to have a solution "soon" (whatever that means). I suspect that a groundswell of interest (especially from people who are already customers of theirs) might help the solution arrive sooner.