Well has nothing to do really with where it checks or where you are but what hardware it is. While yes, by name these are the same phones they are different hardware / software versions made.
You will not get the update for that phone until released for that specific model.
Also if you are unlocked bootloader and root access but stock rom and recovery you should still get updates to the phone.
Remember that the T-Mobile Sensation has some T-Mo specific features in the ROM, namely wifi calling (as well as bloatware). Supposedly HTC sent an update to T-Mobile, who was not satisfied with it (don't know if it had bugs, or just wasn't as smooth as it should be) and they sent it back. While the delay may be frustrating, it's much less so than a buggy update.
Thanks for the speedy reply, theartiszan. To be honest, I was suspecting something like this (different hardware models selling under the same name), but thanks for confirming that. Also good to know that tampering with the bootloader won't hinder any update mechanisms. The ROM is still a stock T-Mobile ROM.
@jaylanphnx: true, a buggy update might be even more frustrating.
Sorry for the confusion regarding the ICS update. Both theartiszan and jaylanphnx provided some great info for you here, but if you'd like to discuss the update more, you can post in the main ICS thread which I've linked below.