As Twitter has recently taken aggressive moves that clash with some parts of the developer community, thanks to their acquisition of Tweetie, and creation of an official Android client, third party clients are trying to find a way to drive differentiation, opening up their doors to data beyond Twitter. Some, like TweetDeck, have opened up to LinkedIn. Brizzly serves as a Facebook client, as do many others. Now, Seesmic, at least on its Web version, is adding Foursquare. In theory, the more services offered, the less the reliance on Twitter, despite its dominant position.
You can see Foursquare updates alongside Twitter In Seesmic Web.
If you have a Seesmic account, and a Foursquare account, connecting the two is very simple. Go to the Seesmic Web interface (at http://seesmic.com/web/), log in as you, and then click your settings, in the top right corner. You can "Add a new account" for Foursquare, as well as Ping.fm, if you prefer, and the service will connect, using OAuth, to ensure it's actually you.
Foursquare is listed as one of your accounts In Seemic Web's preferences.
Once completed, Foursquare can be displayed as a single column in your Seesmic Web.
With Foursquare having great visibility, integration of the service into products like Seesmic seems like a foregone conclusion - and I have already talked to other Twitter clients who are going to do the same shortly, but Seesmic Web looks like it was first to the game. If Foursquare is important to you, set your location to Seesmic Web and become mayor of your Web browser.