I'd have to agree with mark on that. According to the official blog of dreamhost, the amount of drive space being used has grown 300% since they quadrupled disk space allotments. This means more data, more bandwidth being used, etc.
That kind of growth can kill a network if they're not ready for it. It's going to take more drive space and a change to the network infrastructure to accommodate it.
Unfortunately, servers and bandwidth take time to get, configure, and roll out (try calling Qwest or Sprint for help with your T1's some time... the experience will drive you insane).
I'm sure they're working on it and things will get better.