Today I updated my HTC Desire to Android 2.2. The update was successful but I noticed one problem; my mobile internet didn’t connect any more. I found out that I had to add (or delete and add new) APN carrier settings in: Settings → Wireless & networks → Mobile networks → Access Point Names.
For a list of settings of Dutch and Belgium mobile network operators see this page.

Hi Marthijn,
Friday I updated my wildfire to 2.2 and experienced the same problem: No mobile internet connection anymore. With other less restrictive search criteria I luckily found your site and also the solution to solve this issue. In the previous days other sites I found recommended clear storage, reset phone bladibla solutions….
In my case I only needed to add the username and password to the existing APN.
Thanks and a happy newyear!
Dennis Nieland