Does rooting a device and installing a custom ROM SIM unlock it as well? I'm a bit confused about this topic. I have a T-Mobile myTouch 3G which I've recently replaced with a Nexus One so it's kind of standing there. I was contemplating rooting the myTouch and installing an alternative ROM but I also wanted to unlock it so it can be used with other carriers.

Now, the real questions:  When rooting and installing a custom ROM, will that also unlock the phone so it can be used with other carriers? Or is that a baseband feature?
No, rooting your phone will not let you use it on other carriers. If you have a good standing account with tmobile, and I believe you have to have been a customer for like 9 months (or something like that), you can call them and tell they you are going out of the country and want to be able to use an international sim card. they will give you the code you need to unlock your phone so you can use it on other carriers. 

You will not get 3G data speeds unless the carrier uses the same bands that tmobile does. for example, AT&amp;T does not use the same bands, so you will only get EDGE.