SWMM has been updated again (some months ago, actually). The biggest change is that it now allows for 'LID controls'. A couple of builds ago (5.0.019, 2010 July 07), as stated in the Update History, "The ability to explicitly model five different types of Low Impact Development (LID) practices at the subcatchment level has been added." Basically, you now now have the ability to model retention (infiltration and evapotranspiration) in storage and conveyance elements. That is a significant modeling capability addition.