What an ordeal! In order to backup the CalDav App from my HTC Desire I had to go through a TON of tutorials and Howtos knowing that if I screw up, chances are my dear phone will get bricked. So after 2 hours of reading, ISO burning and flash galore, I got a fully rooted(Secure flag OFF) and unbranded HTC Desire.
In case I have to repeat this I am posting these links that did the trick (German only, sry).
- Creating a Goldcard for temp ROOT access on branded devices
- Rooting via unrevoked3 and recovery-RA-desire-v2.0.0 ROM
- Flashing with AlphaRev. S-OFF (security off) thus disabling NAND security