I just got my TouchPad 4G today and turned it on (with the AT&T SIM installed) and connected to my wireless network. I created a new webOS account, etc.
I then noticed "AT&T" as the carrier and got the notification about adding a data package. Tapping the notification took me to the AT&T Communication Manager. I then took out the AT&T SIM and put in my T-Mobile SIM card only to see "No Service" in the status bar.
I went back into the AT&T Communication Manager (after I tossed the card away) and it then began to initialize. After 30 seconds or so, it said activated and "T - Mobile" showed up where "No Service" previously was ("AT&T" took this place holder when said SIM card was installed).
I now have an "E" for EDGE next to the signal bars (come on 3G roaming agreement and/or 1900 MHz refarming) and am able to use the AT&T Communication Manager to track my data usage (not that I care as I'm grandfathered in the $5.99 T-Zones unlimited internet).
While it's clear that I'm able to use T-Mobile's network without an issue, what is weird is that I had to "activate" it using the AT&T app. Had I not, I'm pretty sure it would have been stuck at/on "No Service."
By the way, I'm running webOS 3.0.1.
Any insight would be appreciated.
EDIT: Interestingly enough, the internet and MMS APN settings were automatically populated.
Internet used "wap.voicestream.com" with a username of "Handspring" (lol, no usernames or passwords for T-Mobile) and a password that I'm unaware of (it's encrypted).
MMS used "wap.voicestream.com" with a username of "hs_null" and an encrypted password. An MMSC of "http://22.214.171.124/servlets/mms" and a max size of "300."
I'll compare these to known T-Mobile settings. I know that "epc.tmobile.com" can be used, etc.
EDIT #2: Some pics: