Despite my somewhat flippant previous repsonse, this is actually a serious subject. I might not think much of the idea of viewing web pages on mobile phones but it is the next big thing. It might also be, as in Greg's case, the thing which really triggers the move to web standards.
I'm tempted to have a go at a mobile stylesheet for PunBB since it should be fairly easy. For example, viewtopic only displays in two columns because there is a float. Take away the float and the user information moves above the post. Take away all of the user information apart from their name, kill avatars/smilies/signatures/ and you have a perfectly good format for a PDA. What a PDA would do with the scrolling boxes I have no idea.
The only fly in the ointment I suppose are data tables, a tableless design is naturally more suited to a narrow format.
Fromt the point of view of implementation. I think the way to do it would be to sniff the user agent size and then, if narrow, serve a special stylesheet instead of the normal stylesheets. I dont think there is any need to give users of PDA's a choice of colour schemes etc provided what they get works.