When you use unrevoked 3 it flashes a custom recovery on your phone (ClockworkMod), from my understanding is the OTA will not successfully install if you have a custom recovery. To receive the OTA you would need to flash the original recovery back on your phone. Make sure your phone is S-Off before doing this!
You can get the original recovery image here:
[Recovery] Stock 2.1 Recovery Image for DI - xda-developers
Download the PB31IMG.zip and put that on the root of your sd card and boot into HBOOT (volume down while pressing power button) and that will flash the original recovery on your phone. You will loose root privileges while you have the original recovery, after the OTA installs you can flash a custom recovery back on your phone and get root privileges again.
unrevoked forever has the steps to do so:
public:forever [RootWiki]