It is still a rumour, but if you can wait a while see
Motorola Sholes Android phone headed for Verizon?. Android has a couple of ebook readers, FBReader and Aldiko, but they are both for DRM-free ebooks only. B&N will probably release eReader for Android at some point.
Both MobiPocket and eReader (also now a B&N version) have Blackberry apps. I think is a telling that B&N have not made a B&N version of the eReader Windows Mobile app. There is good reading software for WinMobile, but most of it is "legacy" (solid software, but not likely to get upgraded much).