Many people, when logging into their Gmail accounts are seeing this:
Attempted to login to my gmail account [name here] on [date here].
Username / Password do not match.
One individual documented why he thinks it happened to him in this thread.
In any event, Google has responded saying:
I'm sorry for any inconvenience that all of you may have experienced. I am very glad to hear that Hijacked has regained access to their account. For those of you that have yet to regain access to your account, we have contacted you via email to assist you further.
Please rest assured that we thoroughly investigate all reports of compromised accounts. We take account security very seriously and these types of issues receive high priority. Any kind of feedback you can provide us with is extremely valuable, so I'm forwarding everyone's feedback to the appropriate folks on the Gmail team. We work very hard to provide a great user experience on Gmail and are always working to make it better.
Update: DaveN has a screen capture of the hack and describes how this puts your AdSense account at jeopardy as well as your client list.