How do I unlock an Optimus T? Is there a "vanilla" firmware I can use? I'm an experienced iOS developer, and I just bought my first Android phone, used, on eBay. The phone is carrier locked and I'm looking to swap in my iPhone SIM card. (I have an adapter.)

The phone is carrier locked. Is there a way to root or erase the firmware to unlock it, or must I use an unlock code? If so, what are my options for obtaining said code?
Unfortunately it appears your only option is an unlock code, I've just done a search.  This XDA thread has a couple ways to get into the unlock screen so you can enter the code.

Your options for get the unlock code are to ask the carrier the phone is locked to -- they might give it to you if they're feeling nice -- or to buy it online.  There are no reputable sellers of unlock codes online, of course, but you could search around and find someone whose experience seems reliable and use the same service that they did.