Step 1 - Before You Root

First, you need to downgrade your ROM to 2.73.405.5 (Nwork dependant)     

Confirmed you can still downgrade if you flashed the official GSM 2.1 OTA Update

Sprint CDMA Hero Users - Use this guide on XDA to install a recovery image
Orange UK Users - Use this HTC Program to downgrade to 2.73.61.5
Orange Spain Users - Just skip this step, it should work for you.
Unbranded Users - Use this HTC Program to downgrade to 2.73.405.5
T-Mobile UK (G2 Touch) Users - You need to do it the hard way, use this guide.
Anyone Else - Try rooting, but if you get errors in step 2 then use this guide.

Once you've done that:
Step 2 - The Root