here's to hoping that the shadow in the future will actually be drawn by plasma or compositor, instead of being part of the packground pngs. Roll-your-own shadow was an annoying visual problem in SK, and I really hope Plasma moves forward on that one.
being tired (it's 01:00) and not really feeling up to any serious hacking, i took stackblur in hand and came up with this:
the shadow there is done based on the shape of the item (generically, not specifically for that shape). the png's shadow is on top of it (i didn't both lopping that off; this was a "i'm tired, let's do something quick and fun" hack remember ;) and if you look closely you'll see that the shadow seem sto be cut off at the bottom a pixel or so prematurely. performance wise it's quite fast, but could be faster yet if i bothered to do a few smart things with it.
in any case, i'm not going to commit this to svn (well, i am, but #if 0'd out for future reference only) because i know we (or rather zack, frederik, etc) can do even better with a little effort.
but you can see it's possible and pretty trivial with stackblur in hand (9 lines of code, to be exact).