I had been running various builds of CyanogenMod on my Thunderbolt, on the MR 1 radio. Sometimes on GPS, it would find my location, sometimes not. If I was just relying on the cell towers, it was putting me in Greece (oddly enough, it didn't think I was roaming, though). CM7 just came out with an update a couple days ago, so I'm running the CyanogemMod 7.1 RC 1 right now, and I updated to the MR 2.5 radio also. That fixed all of my issues.
I would recommend updating to the 2.5 radio. There are updates available for rooted AND unrooted devices, though they're different files you have to download and flash.