My guess is 4 to 6 months for the 850/1900. Good news is the price will drop by then. I think the only way you see that 1700/2100 is if T-Mobile signs up to pick up the phone, then maybe Nokia would make it in a tri-band HSPDA 900/1700/2100. Wouldn't hold my breath for that though since T-Mobile in U.S. is still implementing 3G 1700.
The reason I was hoping 1700/2100 is because the infrastructure is already laid out among major cities. They just decided to turn on UMTS/HSDPA only in NYC, but I heard it's gonna be turned on among major cities, like Atlanta, Dallas, Chicago by the end of the year. They're just not making a big deal of it, preferring to do it quietly.
Actually, even if they added 1700 to the European version, it still wouldn't work with T-Mobile's 3G network. The 2100 frequency that T-Mobile uses is not exactly the same as what is used in Europe, so there would have to be a fourth version made (assuming that Europe, Australia and US AT&T all get a version). I don't see that happening.
That sucks but I don't think there are enough of us who use T-Mobile that would actually be willing to shell out the money for something like the N96. That sucks even more because I'd really like to make it my next phone. I probably will anyway, assuming the slider is more solid and the battery better than the N95.