Hundreds of Google AdSense publishers cannot login to their AdSense accounts right now. Derek Chew showed me that when he tried, he ends up at a URL that looks like this https://www.google.com/adsense/legacylogincallback. The error when you land at that page is "UsernamePasswdNotMatch." I personally can login to my account, but many cannot.
First reports came in at about 1:07pm (EST) today and the issue seems to have lingered on for many, until at least the time of this post.
There is no official word from Google on this login issue. I do see several threads at Google Groups the AdSense forum, with complaints.
One user said the trick is to first login to Gmail and then try logging into AdSense. Give it a try.
Update: At about 3pm (EST), an official Google representative wrote that they are aware of the issue. She said the issue should be fixed within minutes. But then a few minutes later told people to use the workaround, which is to login to a different Google service using the your Google Account (Gmail, Google Reader, Orkut, etc), you should be able to access your AdSense account just by going to www.google.com/adsense.