FYI --- Looks like Gmail (& Google Apps for your Domain) is currently down for most people... though Google hasn't acknowledged it yet...
Update @ 11:10 AM CST
It appears that some of the DNS settings for Gmail had changed, but some DNS servers didn't get the information, and didn't propagate correctly. Manually "Flushing" your DNS seems to fix this issue:
via Command Prompt with Administrative Rights, type "ipconfig /flushdns" then press enter.