The design of WinPhone 7 pretty much allows an arbitrary resolution, it's designed to be fully scalable. 1920x1280 @ 262 DPI @ 4 inches would make one ridiculous high res screen

As for what's in store, I wouldn't be surprised to see a large-format screen device like a tablet in the months following WP7's release.. but I don't have any specific knowledge of one yet.
Support my work! The more phones I can get my hands on the more ROM goodness I can churn out. Like BUTTAH