"We're aware of a problem with Google Mail affecting a majority of users. The affected users are unable to access Google Mail," mentions Google's Apps Status page. The message was posted at 0:53 pm PDT and Google promises to solve the problem in less than an hour.
Gmail's web interface can't be accessed, but you can read your messages and compose mail if you use Gmail's iGoogle gadget. Another way to connect to Gmail is using a desktop mail client: POP3 and IMAP come to the rescue.