There's already been a second set of seasonal races, this suggests that maybe they will continue.
But the extra money and experience points for A spec races will stop at the end of the month, so I'm guessing if new seasonal races appear with a finish date after 1/2/2011 then this might be something that Polyphony will continue for a while.
Kaz has mentioned that there are 'plans' for GT5 but no details have been released, I think it's a case of wait and see what happens.