C.F.
Hi Alex!
I'm a WOTH mom, so I can't do playdates, but I think that the reason why you're not seeing requests on this site is because things in this area are *so good* - there are tons of ways to get into playgroups.
Here are a couple of ideas:
Mothers of Preschoolers http://www.mops.org/ - generally meet in churches and there are a bunch of groups in this area. Use the search box on the left to search by zip code to find the ones closest to you.
Our Kids http://www.our-kids.com/ - a site constantly updated with activities and events for kids in the metro-DC area. They have a community board (great way to find playgroups) and offer discounts for ticket events.
Meetups.com http://tinyurl.com/dg2zgr - this link is to the results of a search I did on the main page of the site using my zip code and the term Playgroup. The list is of over 95 groups. Several that popped up on the front page, I've been active with (a while ago.)
A Parent in Silver Spring is a fun blog to keep an eye on: http://www.aparentinsilverspring.com/2008/06/100-things-t...
Finally, look in your neighborhood - do you have a newsletter or website you can post a request for a playgroup to? Using the library is a great idea as well - ask at your local branch if they have events geared to preschoolers (a great way to meet others) or if you can post a request for a playgroup on a community bulletin board or something there.
Like I said, there are tons of groups operating in this area - your biggest problem will probably be narrowing the field down. = )
Good luck!