It will work on T-Mobile because it is quadband GSM. The Band IV (1700/2100) coverage is LTE, not WCDMA, so nothing faster than EDGE data on the T-Mobile US network for the Lumia 900 as it is currently configured. There are variations of other Nokia devices, such as the C5, where one variant has Band IV WCDMA and others do not. It may be the Nokia 900 could have a variant the will have Band IV WCDMA support. If not from T-Mobile perhaps Wind or another Canadian vendor might get it.
According to the http://www.developer.nokia.com/Devices/Device_specifications/Lumia_900/ it has
WCDMA Band II (1900)
WCDMA Band V (850)
WCDMA Band VIII (900)
LTE Cat3 Downlink 50 Mbps
LTE Cat3 Uplink 25 Mbps
Nokia does not specify the bands used for LTE.
GSM 850 / 900 / 1800 / 1900 (quad)
LTE 700 / 1700 (dual)
WCDMA 850 / 900 / 1900 / 2100 (quad)
So, actually, it isn't pentaband for any given encoding scheme being used. Note that phonescoop includes LTE bands and an additional WCDMA band when compared to Nokia's information. Hopefully the Lumia 800 for NA will be pentaband as the Lumia 900 is going to be slow data device for a T-Mobile user.
I just created a posting to ask for this device, I wish I had of seen ur posting b4 I posted asking about this device....I just want to be able to have an ALL-IN-ONE Windows phn to xfer my music from my Zune player to hopefully upgraded NOKIA Lumia 900 which have all the memory I need/want and to be able to expland the memory, that's why I didn't upgrade to the NOKIA Lumia 710 b/c it does not have enough memory or not able to expand the memory...that sucks! I really do hope T-MO does get this phn very, very soon, I'm sooo ready to upgrade!!