I have just been through this with another user privately and it turned out he was given a window where you couldn't view the full key.
As Cheer has pointed out you should sign up for your key at: http://code.google.com/intl/zh-IT/apis/maps/signup.html
You should enter your domain name without www.
When you have signed up you should get a page with your full key in view.
As you can see the string is very long. Also as I have mentioned before you must be cautious of not allowing a space at either the front of end of the string when cutting and pasting and don't allow a second line (paragraph).
Even if you have already signed up for your domain API key, you can always head back to the sign up page and enter your domain and email address and it will then take you to the page that displays your full API key.
there I log on my google account and activate the Google Maps API v2 service.
The API key i get from the API Access page, in the Simple API Access section.
On both keys still have this error.
Google had to change something because before this error was not there. Appeared about January 2012.