If you're using FireFox, make sure you clear ALL of your cookies (for those more technically adept, you just need to delete the dreamhost.com cookie.. I personally had two of them for some reason). Once your cookies are cleared, hit ctrl+f5 and, since someone mentioned this worked for them earlier thought I personally didn't need to do it, close and re-open FireFox. You should be able to login to the panel after that.
If not, then something is going wrong with the panel and you might have to wait a while I guess.
Chips N Cheese - Custom PHP installs and the like!