I took delivery of my new iMac 27" on the same day. I've never owned a Mac before, and when I had imported all my email, contacts and calendars from my Google account, I expected email addresses to come up when I started to type names in the "To" box in my emails. When this didn't happen I thought it must be something I'd done (or failed to do), but this report suggests there may be a glitch in the integration of Contacts into Mail,
Although it may have been done, try downloading the 10.8.2 Supplemental Update to see if that helps:
Try logging into another account, either by creating a new one or using Guest and see if the behavior is the same.
If you have loaded any 3rd party software, try booting into the Safe Mode to see if you get the same problem:
Also, you might try running some basic maintenance tasks since they can't hurt:
Run Disk Utility/Repair Permissions. Repeat until it shows no repairs were made. Clearing the log in between runs can make the process less confusing. Also see: Disk Utility Repair Permissions You Can Safely Ignore
PRAM reset: http://support.apple.com/kb/PH11243
SMC reset: http://support.apple.com/kb/HT3964
Below is a Mail 'all-in one' with links to various troubleshooting Apple support articles:
The 10.8.2 Supplemental Update seems to have solved the problem.
I say "seems" because I was asked to restart my Mac once the installation had taken place, and postponed doing so because I'm in the middle of transferring a large collection of music files from another computer, and didn't want to interrupt the process. Should the update have worked without the restart?
Anyway, having confirmed that my iPad's versions of Mail and Contacts behaved correctly, offering me a progressively smaller selection of email addresses as I typed in a recipient's name, I tried again on the Mac, and got the same result.