The best way is to s-off your phone with unRevoked forever. Then flash back the stock 2.1 RUU image. It's not difficult but you have to have s-off or it won't flash the radio back.
Get this:
UnRevoked Forever
Download and install it on your phone.
Get these files:
Stock 2.1 PB31IMG
Place it into the root of your SD card.
Reboot your phone into hboot by pressing volume down + power.
It will find the PB31Img and you will see a blue bar come up on the right hand side of the screen. Eventually it will test and verify it and read text will come up asking if you want to install. Hit volume up to install. Let it do it's thing. Then reboot.
You will now be back at 2.1 stock non-rooted.