3G networks are broadly divided into two ranges of spectrums.
a) The 1700MHz and 2100 MHz bands
and
b) The 1900MHz bands
For instance, in the US, T-mobile listens on the 1700 MHz and 2100 MHz bands for 3G network.
AT&T, on the other hand listens on the 1900 MHz band for 3G network.
What this means is that the phones are not inter-operable in 3G between these two carriers. The AT&T iPhone, even if unlocked, will only work in Edge network in T-Mobile.
Likewise, the original T-Mobile Nexus One will only work in 3G in the T-Mobile Network. It falls back to Edge in AT&T.
Looking at the spectrums that were auctioned in India, it appears that they are all in the 1900 MHz band.
Hence, any 3G phone that works in the AT&T network in the US (1900 MHz) will work in 3G in India, whenever 3G goes mainstream. Which sucks since a lot of the great HTC and Samsung phones are in the T-Mobile network 
.
If you think otherwise, please leave a comment.
Update:
Some confirmation for this from a thread here:
Alright i just got off the phone with nexus one support..
The T-mobile version (900/AWS/2100) WILL NOT GET 3G speeds in India/Asia/Europe since it requires the AWS band for upload which is not present in India/Asia. In india we utilize the 1900 band for upload which is clarified in the 3G spectrum auction.The At&t version is the one which will get 3g speeds in India and Europe which has the 850/1900/2100 band.
The specifications for the iphone 850/1900/2100 are the same and we can get 3g on it in all the asian countries and europe.
I have confirmed this with Google.So the AT&T version is the one to buy if u will be using your phone in India/Asia/Europe for 3G
Hope this clarifies the discussion.
