short of doing what MArtin suggests there is no way of finding out. Each post does have its own numbered ID but as it also acounts for all the posts that have been deleted - it is slightly out due to all the spam posts.
Lets just say anyone that posted today can share the glory of the 30 000th post. So go on do a little dance in front of your computer and chant 30 thousand, 30 thousand, 30 thousand oooh ar! Especially those of you at work
