SPIT defined as "SPam over Internet Telephony" could potentially be a a problem in the future. I've talked about VoIP spam in the past, i.e.
Get Ready for Nigerian VoIP Spam!

SPIT will certainly become more of an issue as networks converge and their is native 100% VoIP-to-VoIP calls that never touches the PSTN.

Here are some other interesting articles on SPIT you should check out:
Spam, DoS Headed VoIP's Way
Watch Your Mouth