While I highly doubt they will have every tower in the entire country switched over by the beginning of next year, it should be a large majority of their markets - of course, they're focusing on big cities first and working out from there.
This is from May, but it shows that T-Mobile will be confident enough to try and pull in iPhone users by the end of the year:
T-Mobile CTO Says Network Will Be ?Should Be Ready? For iPhone Users By Fourth Quarter | | TmoNewsTmoNews
I'm confident enough in this to be betting on buying a 920 unlocked (unless someone comes out and surprises me with a 3.7" WP8 device or Lumia 800 clone before then), since I'm with T-Mobile myself.
Yes.