For our purposes, you can think of a SIM cards as basically holding your the account/service info. They are portable from device to device.* Non-SIM phones associate the phone with your account/service.
Any TMO sim can be used in the Optimus T. Example: you can borrow a friend's TMO sim and pop it into your phone and it will work fine immediately.
Any GSM SIM can be used* if the Optimus T is subsidy-unlocked. You can buy an unlock code off eBay (usually less than $10) to use a ATT sim, for example.* If you are an existing TMO user and you tell them you are travelling to Mexico they may give an unlock code for free.**
If you are going to be in Mx for a while it might be cheapest to buy a local prepaid SIM to call local Mx numbers.
If you will have access to wifi sometimes you might skip the local SIM and use wifi:
1. to make VOIP calls back to the US using a SIP client or GrooVe app.
2. to use Google Voice for SMS purposes.
3. for email
I think that's what I'd do.
When I go out of country I take out my US SIM because data/sms/voice calls outside the US are generally outrageously expensive. Nightmare stuff. People have racked up multi-thousand dollar bills while unintentionally roaming, particularly on data. You have been warned.
It's late and I don't know that I'm making much sense. Perhaps someone else will chime in with better or more helpful info.
* barring technical issues like frequency bands, etc. It is common to not have full use of another carrier's data if the frequencies are unsupported by your phone.
** sometimes based on length of time with the carrier
