I think the biggest problem here is people are passionate about their own causes, which is a good thing, but makes it hard to get a consensus!
I think as Mel said anything based on politics/religion would probably be good to stay away from in terms of generating mass enthusiasm.
I think cancer's a good one (hey, all of these sentances start with 'I think' heh), beause it's pretty universal, there's very little politics involved, most people here have been affected or know someone affected by it, and it's also related to the show as Breast Cancer Research was Lauren's nominated charity on the poker thang.
That's just my two cents.
This thread is somewhat dead anyway *sniff*