Unlock

From CyanogenMod Wiki

GSM phones are often locked to only work with the sim cards of a particular carrier. The G1 and MyTouch are locked to T-Mobile (in the US at least). The ADP1 and ION are not sim-locked and should work with any GSM carrier's sim card (though 3g data depends on country/bandwidth). If you buy your phone from the manufacturer (e.g. Samsung) rather than the network (Vodafone etc.) it will be unlocked. It will probably save you money too, allowing you to get cheaper SIM-only contracts (or a PAYG SIM).

Obtaning root access on your phone does nothing to the sim-lock.

The only way to unlock your phone to all sim cards is to obtain a subsidy unlock code. These can be obtained from your carrier (T-Mobile US gives them to anyone over 90 days into their contract, other carriers don't) or from companies on the internet that can generate such codes. Such internet companies are of questionable repute.