I've thought about that before, but I think that there might be legal problems with that. It might make the forum a "money transmitter".
Surely not with changetip? Could that not just be implemented? I think I'd like to see this feature as long as it didn't cause any problems like people have already mentioned.
I think adding changetip itself to the site would work without having to be a money transmitter. Maybe just don't make it a site function, that way a third party is taking care of all the actual bitcoin movement.
But I can understand Theymos reservation with not implementing it.