I've always agreed that using the tipping or the charity metaphor is a bad idea. Unforuntately the media loves the idea of the Tip Jar even though no one else does (but it is useful for explaining the concept to someone new to voluntary payments).
So over at Fairtunes we're having a vote on a new name. Currently 'Voluntary Contributions' is winning. I think one of the more interesting suggestions as been to call them IPs.