You’re question is not the first time I’ve heard it and I might have an answer for you.

I got off the phone with T-Mobile (to clear up some activation issues with Sidekick) and I asked them about this whole “unlocking” conspiracy. The guy told me that there was no honestly no actual locks to the phone. There was just SIM locks on the cards.

Based on what he told me:
If you want to unlock your Sidekick 2, you’d have to insert your Cingular simcard where you would insert a T-Mobile SIM.

Next, a screen would come up with the words “Waiting for activation”. Hold the Menu button and the letter “L”. After a few seconds let go, and it should take you to the jump screen (or, the main menu) . You should be able to make calls and texts and the SMS features.

Reminder: You will not be able to use any of the internet services, AIM, etc (unless your Network Band supports frequency is 800/1900 MHz if you live in the United States or Canada; and 1800 in Asia and Europe). According to my research, Cingular/AT&T uses 850 and 1900 MHz, but apparently those who’ve used their Sidekicks with Cingular are not able to use the internet service, so I believe this depends on the coverage in your area [don't quote me on this]

If this requests any SIM locks (more specifically a three digit number to unlock the SIM) then you’d have to try a combination of numbers from 000 to 999. Or you could just buy a new Cingular sim card. [don't quote me on any of this section here .. this is only if you get a request, which .. shouldn't really happen].

If you got to the jump screen and your service is still not working. You can try going to the Settings menu, and selecting Network Options. From there you should scroll down to find “Network Selection”, you’ll probably be given the option to scan and select the network you’re choosing. After it’s done scanning, choose Cingular. You should be fine.

Really, that’s all there is to it. The guy I spoke to from T-Mobile was very confused that people believed there was an acutal lock to the phone. All there are locks on are the SIM cards (both T-Mobile and Cingular might have them) and Privacy Locks on the Sidekick (if you found the phone, it was stolen, or you bought it from someone on eBay who still has their name registered on the phone).

I hope this helps and I hope you solve your problem soon.

EDIT:
Call T-Mobile. I just spoke with someone at Customer Service, and they said they would be glad to resolve this issue with you.