The update is rolling out today, and it does exactly what you would expect. When you're browsing for a place to eat, if the restaurant you find is part of the GrubHub Seamless network (over 20,000 strong at this point), you'll be able to tap on the GrubHub or Seamless icon (the two companies merged back in November, but don't yet have a unified icon) to place your order.
Foursquare notes that the service is so easy that you won't even have to put on pants (though that is recommended protocol before the delivery person arrives with your food.) The update also brings better maps to let you dig in to specific neighborhoods. The update is coming today to Android and iOS, but there is no word on when the update will be making its way to Windows Phone.