All cellular handsets have at least two quite powerful magnets inside already, the speaker and the ringer/MP3 speaker!
Anyone who says that using a magnetic case will cause harm is talking complete and utter nonsense! The only issue with magnetic cases revolves around slide phones; in such instances the keypad lock unlocks if the handset is placed the wrong way round in the case. The Nokia 6280 was a classic example.
There are no components that could be damaged in your cellphone. At the end of the day you ARE risking destroying your phone by not using a cover!
I'm in the trade and I couldn't even begin to count how many such cases I have sold and the only issue that comes to mind is the Nokia 6280 keypad lock and sometimes causing yourself an injury when getting into a car on the passengers side (assuming the case is on a belt on the left) if it catches on the door handle!!
If you find that your keypad lock unlocks when in the pouch, simply place the handset the other way around! Remember back to school and magnets having north and south poles?
The compass in the Desire would be affected but only while in the pouch, I guessing you don't have x-ray vision and would take it out of the pouch to use it?
Your biggest worry with any case is 1. It will last at least as long as the phone and 2. It doesn't clash with your belt colour!
