As of right now, they're saying that no, it cannot be upgraded to WP7, but hopefully that will change due to popular cust demand. Plus, there are already hacked roms that allow WP7 to be loaded onto the phone.
I've asked T-Mobile this very question a few times while on the phone & they tell me it can once it is out (Win 7). BUT my guess is I am talking to a salesperson with little to no accountability? anyone know otherwise?