Not sure, but carrier lock means that you cannot use a SIM that is not from the carrier that it is locked to ( like the iPhone ).
Carrier unlocked does not mean that it will work well with other US carriers, just that i can work with any SIM.
Point is you can take it anywhere in the world there is 3G coverage (at the right frequencies) stick a SIM in it and it will work. Not so with the iPhone (unless you bought an unlocked one, which is not possible in the US).
T-Mobile and AT&T 3G services require different antennae (and possibly chips). You can use the same device to access their GSM networks and get onto Edge, but when it comes to 3G a device either supports one or the other. To my knowledge there are no phones (or devices) on the market that support both AT&T and T-Mobile in the same device.
IIRC, that's why there is a Nexus One for T-Mobile and a Nexus One for AT&T. You can use either to connect to GSM/Edge, but you need the carrier-specific model to connect to 3G. I believe it's a band issue, but I haven't read a whole lot on the topic.