SIM lock

some nasty way whereby mobile phone (cellphone) operators can lock your phone so that it can only use SIM cards from their network, or worse, the specific card that the phone was sold with.

you can usually get a code number from your operator to release the lock on payment of a fee.

SIM lock removal requirements imposed by different GSM providers in the US:

AT&T Wireless (now merged with Cingular): Will not remove a SIM lock from your phone for any reason. The Terms and Conditions of AT&T Wireless (and Cingular) state this - though they are very vague (and I quote): "...if your Device utilizes a SIM card it will only accept a SIM card provided by us...". This seems to indicate that the phone was made specifically for AT&T Wireless or Cingular. But a few of their phones are also available from T-Mobile.

T-Mobile: After 90 days of active service, T-Mobile will remove a SIM lock. You just call or send an e-mail containing your subscription and phone information and they'll send you a code to disable the SIM lock. By far, T-Mobile is less restrictive than most providers.

From this, it appears that the only GSM-based provider that removes a SIM lock from their phones is T-Mobile (and then only after 90 days of continuous service).