Can an HTC phone be unlocked?

Yes you can unlock HTC phones and use it with any GSM network. I got mine here http://www.unlock-zone.com/unlock-your-phone/rs5wp... took about 30 minutes and their support staff was very helpful.

Well, most other phones are easy to unlock. All you need is an unlock code from the manufacturer.

With T-Mobile, if you buy a phone from them, your account has to be active for 90 days before they'll help you get a code.

Now, the more important question is why would you want to unlock a smartphone from T-Mobile (I'm assuming you're looking at something web-enabled)? You will lose the ability to get 3G or better networking if you do that because AT&T and T-Mobile use different 3G data frequencies. Just sayin'.

By the way, if you don't care about paying full price for the phone, the Google Nexus One comes unlocked by default (it's made by HTC and is sold by Google). There are also two versions of the phone, one for T-Mobile and one for AT&T. I have the T-Mobile one and it kicks butt.