I'm not sure if it's really been tested; myself for my forums I can browse them with my PDA (PPC2002) using the built in IE browser; but that's a given as it's just a web browser; though even with it i've had issues, such as no java support.
Myself I've been giving strong thought to creating a feed for my forums for "small devices" (cellphones, pda's, etc) without having to "fiddle" with modifications; step one is to fiddle the UBB.News modification at UBBDev (a file really, it's not really a modification as it doesn't touch any stock files) to be RSS2.0 compliant so many readers and relay programs can cache the content... But this is more of a 3rd party's 3rd party kinda deally <img src="https://www.ubbcentral.com/boards/images/graemlins/wink.gif" alt="" /> ...
For blackberry support it'd be kind of interesting as you'd really have to own one in order to test compatability with one...
