Afaik, in theory, (with no 301), if the PR of the link is high enough, Google will list site.com as the URL in the serps rather than www.site.com and it doesn't make much difference, you're still there, but in practice this isn't necessarily the case. I don't know if some dupe content penalty kicks in or what, but when it happened to us, it knocked our index.htm out completely until we got it taken care of and their site had been re-crawled with the right link.
I don't want to start a big deal on this, but it's best to watch out for how people link to you and make sure it's with a consistent URL form. I never bothered with a 301, so I don't know if that can be a source of problems, (it's not supposed to be), but since our little adventure, I look at the logs very, very closely. I don't want anything slipping through and repeating that mess.