There's a fairly comprehensive US Govt report on VOIP security that also includes a lot of good background info on VOIP in general available at

http://csrc.nist.gov/publications/ni...0-58-final.pdf

There's also a Slashdot article that discusses the report at

http://it.slashdot.org/it/05/05/06/0...id=215&tid=218

Worth reading I think.