I have never come anywhere close to my limit. I think the highest I've ever gotten was a bit over 100 GB a month from the sites, with another 50 GB through my (private) web proxy (I have a bad Amazon tv episodes habit). My limit is 500 GB at the moment, so I'm not too worried.

They do give you private IP's so that your "slices" can talk to each other - this traffic doesn't count against your bandwidth quota.

http://www.slicehost.com/questions/#private-ips