For curiosity's sake, I checked out the dates for each of these.
Beginning: October 3rd, 2002
*3 years and a couple of months pass*
100,000 posts: December 31st, 2005
*2 years pass*
200,000 posts: January 15th, 2008
*1 years and a couple of month pass*
300,000 posts: March 19th, 2009
*10 months pass*
It would seem that there is a trend and that if it continues the 100,000 posts marks are going to keep coming sooner and sooner. Maybe 500,000 will happen as soon as this summer...