No, in fact, I think it takes away from the "appiness" of an app. The only one I've seen where it compliments the design, rather than hedges it in and distracts the elegant lines is Zentomino's gorgeous icon (as well as TanZen).
I prefer the icons to have borders and the "shiny" effect with the shading. It makes them look kind of like buttons. Otherwise, they look too "flat."
it all depends on the icon. If you have strong artwork a 1 or 2 px stroke of similar colors wouldn't hurt. The geodefense ( I think ) icon for example is way too simplistic and looks like the game has low production value.
I really don't like icons where they just leave the outside black so it is oddly shaped on the display - with that in mind I do like borders as it discourages that practice. A bigger no-no IMO is having the app name in the icon ... as well as a couple of millimetres below ><!