If we have around 30 new members every month, in 7 months (March) we'll have around +210 members than now, which will give a bit over 400 members by March 2011 (roughly the double of the current number of members).
After the final version of TSoG is released, there will probably be a bigger increase in members.
However, I'm expecting that the regulars will not double in numbers by then (perhaps I'm just pessimistic).
Craig (and I believe KZ) have more experience on these numbers than me because I wasn't a member of the old forums when TRPG2 was released, so I might be wrong.