I have personally tested 4 phones, 2 from AT&T and one from T-Mobile and one purchased unlocked in India. All 3 work in 3G just fine with Airtel.
iPhone4, iPhone3GS, Samsung Captivate (AT&T), Google Nexus S. All these devices work fine in 3G with Airtel. So long as the phone supports 2100MHz, it will do 3G in India.
Update2
I bricked my Samsung Captivate trying to update it to Gingerbread. I bought a Google Nexus S (review coming later), it is a 1700 MHz+ 2100 MHz phone (T-Mobile in the US). This also works in 3G perfectly.
If you are buying a US phone, you can buy any T-Mobile or AT&T phone, both support 2100MHz and they will work in 3G in India.
Update
I have relocated to Bangalore and both my Samsung captivate (AT&T 1900MHz) and my wife’s iPhone 4 (AT&T 1900MHz) work on 3G with Airtel.
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.
