If you're receiving any Google email that you need to link you Google Analytics account with your Google AdWords account, the email appears to be legit. Google Help clarifies as it explains the process of linking your Google Analytics account to your AdWords account.
The email states:
Hello, It's come to our attention that your AdWords account has not been properly linked with your Google Analytics account. If you do not link your accounts by March 5, 2008, we cannot ensure that your data from AdWords will continue to populate in your Analytics account.
It's fast and simple to link both accounts. Please review the various scenarios below and follow the instructions that best suits your needs.
It looks like you have less than a month to do so, but in the end, I think it's worth it.
Of course, if you are receiving the email and your two accounts are already linked, you might want to look into it. One forum member had that problem. But it looks like everyone is getting this email...except that I didn't.
Forum discussion continues at Search Engine Watch Forums.