the ars article incrorrectly puts direct2d as a competing technology for wpf and SL though. (they even claim neither actually use d2d whitch is completly incorrect)

i think where SL will shine is apps where perf is important. SL is still ~2x faster than js atleast in this perticular benchmark (running in ie9beta) and with TPL coming to SL it completely owns js from a platform standpoint

for simple* applications and maximum reach though i think html5/js will be the way to go, but as ars points out, it will be a loong time before html5 is as ubiquitous as html4 is today

--edit--

actually ars in partially correct, wpf is not build on d2d, however ars also claimed wpf and sl are not hardware accelerated and that is completly false. All three fully use directx under the covers including dx11 features like DirectWrite where avaiable