So it took a while, but I can confirm that my iPhone 4 has been officially unlocked by the fine folks at AT&T. It looks like my online chat with the AT&T representative paid off after all.
All I had to do was pop in my T-Mobile SIM and load up iTunes. I was then presented with a beautiful message that stated: Congratulations, your iPhone has been unlocked.
Oddly enough, this unlock has nothing to do with the SAM unlock method that was revealed earlier this morning. This is a follow up to AT&T's promise to unlock out of contract iPhones...