Mindy McAdams has written a useful round-up of current case law (US) and thinking around bloggers' legal responsibility for libelous comments posted on their blogs.
The situation - and defamation law - is much different in the UK where there have been a number of libel cases lodged against blogs and their publishers over the past few weeks.
My collection of internet libel links: http://del.icio.us/Cybersoc/internetlibel
More substantive posts about internet libel:
http://www.cybersoc.com/internet_libel/