Did some research after getting my phone and found that a foreign Du pre-paid card (from the UAE) works fine on the phone and roams on T-Mobile



Network Selection becomes available too.



When using a US SIM card, the phone becomes unactivated and complains the SIM is not valid



Used a friend's AT&T microSIM and an h2o mobile prepaid SIM I bought for this purpose



There is no CDMA/GSM toggle anywhere. With the Sprint SIM in, phone will use CDMA, with the SIM out, it will also do CDMA, when the foreign SIM is inserted it automatically switches to GSM. Not the best way since this means you can't keep your foreign SIM in there if you live on the border, like I do. No biggie, keeping the foreign microSIM in the wallet with a SIM tray tool is easy.

SIM Tray out, back to Sprint:



Tried to use an old Telcel SIM card for this test but I didn't cut it properly or it's been deactivated and I get nothing. Will be trying it in Tijuana on Sunday with another friend's microSIM. Hopefully neighboring country's SIM cards are not locked out.

Here's a video showing how this works