[quote name='heavy liquid']This may be a stupid question, but what exactly does "unlocking" it allow you to do?[/QUOTE]
It just allows you to use whatever sim you would like. For the N-Gage it only allows GSM, so either Cingular or T-Mobile, and some other oversea carriers. So if you have a T-Mobile phone, you can unlock so it is Cingular phone. I don't know that helps.
Think of it this way:
You have one of the black razr's from Cingular. However you only have a T-Mobile sim card. If you placed the T-Mobile sim in the razr it would not allow you to even get to the main menu of the phone. So by unlocking the phone, it would allow you to use your T-Mobile sim in the razr. Pretty much what happens is carriers spend a lot of money to lock the phone specifically for their service, i.e. Cingular, T-Mobile. When the manufactor of the phone releases the phones they are unlocked, which you can do a search online to see that the unlocked phones cost pretty much an arm and leg compared to ones that have been locked to a specific carrier.
Sorry didn't mean to rank, just didn't know if that would help explain it a little better.