I am getting this error as well. I was running CM7.2 rc1 and decided to night to revert to stock to see if I could get the ota.
I used CWM to restore to stock 2.3.3 using the download from gunman's thread. Cleared dalvik, cache and did a factory restore. I noticed superuser was still on there so I flashed jboxers zip to unroot, then used nvflash to restore stock recovery. Cleared cache, dalvik and did another factory reset to be sure.
Checking for an update I don't get a message about whether my phone is up to date or not, I get a message saying it can't connect to the server due to an authentication error.
This happened to me last time there was an ota update. To go from Froyo to GB I had to use the LG tool because my phone wouldn't authenticate with tmo's servers.
It's not a second hand phone or anything. I bought it directly from a tmo store. Dunno why it won't ever authenticate for an ota update.