The Chicago Fire defeated New England 2-0 tonight, giving them a 3-2 aggregate-score win in the series.
Here's the game report from MLSnet.com.
(And I see there's already a nice postgame discussion going on in the post below.)
This means that either Chicago or RSL will fill one slot at MLS Cup in Seattle on Nov. 22. The Western contenders, of course, will be halved from four to two on Sunday.