Ok, I got my achievement today. Here's what I did.
1. Go to http://harmonixmusic.parature.com/ and create an account.
2. Click 'submit a ticket', and then 'submit a ticket' on the next page the loads.
3. I chose the following: Product: Rock Band, Type of Support request: Web, Web Issues: Other web issues.
4. In the details section, provide your 360 gamertag and of course describe the issues you are having with the achievement not unlocking.
For me, I submitted the ticket on a friday evening and had a solution the following Tuesday afternoon. They sent me a message stating the achievement will unlock the next time I load the game, and sure enough, it did. Granted, its not an instant fix, but for those of you who have been waiting weeks or even months for this to unlock, whats a couple more days?
Hope this helps!