go to settings>about phone>system software updates>check now

this should update to 2.2

 

Once your updated to 2.2 (Trusting it's been released on your carrier) just go into your applications and you'll see the "Hot spot" app. Set up the Wireless name and Encryption/Password and you're good to go!

 

Zeus, don't know which country you're in, but here in France you have notify your carrier that you've switched from iPhone to Android so they can make sure you have the proper 3G settings set up.

 

Where did you buy your desire from? Is it locked and running generic android 2.1 + sense or is it running an At&T branded version? As far as i'm aware, at&t don't sell the desire, so updating should be easy.

 

Since you are on AT&T I would suspect you have the Telus Desire from Canada, so do I and am running Froyo 2.2 because I did a flash of the stock HTC Ruu(firmware), since you are on AT&T I would venture a guess that it is unlocked.

There is a Froyo update released by Telus that will probably show up on your phone sooner or later, you can upgrade with that one if you want to keep the Telus stuff on the phone or you can do a upgrade yourself by going to a thread on this forum that shows you how to get the stock Froyo from HTC and further over the air updates right from HTC!!!

http://androidforums.com/htc-desire/148932-froyo-ruu-update-available.html

How you do it is up to you but since I do not have Telus service I did not see any reason to keep the Telus firmware on my phone, plus the stock Ruu has improved my connection to the AT&T Network and also unlocked the 850 band so I can connect to the Edge Network when I am not in a 3G area, Telus does not use the 850 band so it is locked out on the telus phone's for data, voice will still work. You can get voice but the E will not show up at all.

I did not see the part where you said you switched carriers, so you are on T-mobile???

 

Well that is a CDMA carrier, so as of now there is no Froyo for CDMA you need to look at this thread.

http://androidforums.com/htc-desire/189045-its-october-any-word-uscc-froyo-2-2-update-desire.html

You will not have any of that until you get Froyo 2.2 if at all depending what US cellular decides to leave in or take out, you are at the mercy of the carrier with a CDMA phone!!!!! SORRY!!!!

 

I really can't say because I do not know if there are any custom rom's for a CDMA phone, the CDMA phone is not as wide spread as the GSM and so I would not expect very much support from developers as you would find for the GSM. I would say do some Google searching and see if you can come up with something for it before you root because rooting will Void any warranty that you have on the phone. Also see what the other people in that other thread think about it. Hope this helps you some!