Harry Pierson is discussing the correlation between market penetration and hassle free installation experience and hits the nail on its head.
Big market success of Adobe Flash, its omnipresence and high install base can definitely be traced back to flawless install experience. In fact, it's so perfect, that most users don't even know which version they're running. They don't care (me included). And they shouldn't care.
The first thing any platform needs to achieve is simple and effective installation experience. I don't want to manually download platform installers to use certain web services. It has to auto-install and has to be safe. This is what Flash does perfectly.
All current virtual machine based platforms can be installed independently. Or they can be flawlessly smuggled in by a host application setup program. The problem is, that we can't compare CLR (or JVM) based platforms to Windows (as a platform), since the user can't run five of the latter at the same time.
Flash 9 market penetration is currently at ~40%. Flash 8 penetration is ~90%, while Flash 6 penetration is ~96%.
WPF/E will have the same install experience. Bet on it. If it wants to succeed, it will also have to allow multiple versions to coexist gracefully.