I don't know how prevalent SPIT really is, but Grand Central recently deployed first of a kind VOIP spam filter. The press release says:

"(GrandCentral Communications) announced the release of its community-wide PhoneSpam™ Filter. The first of its kind, this free service employs GrandCentral's proprietary technology and its own database of abusive callers together with a user-generated list of telemarketers to filter out unwelcome or insidious calls with a check of a box."

Combined with the company's One Number...for Life service, this free tool can effectively protect not only your internet phone line, but all other phones you have entered into the service.

Sounds nice. Tell me how you deal with SPIT, and do you even come across it?