RIA (Rich Interactive Applications) have this approach that's quite unique, in that they kind of want to sit with fingers in both barrels. On one hand, they want to stay within the browser whilst on other hand they want to breach the browser and live on the desktop.
It's a hard nut to crack as both have merits in choice, on one hand it's nice to sit within the comfort and safety of living inside a browser. Browser's aren't considered to harmful and are generally trusted installed software (i.e when was the last time you opened a browser thinking, "Is today the day I lose my hard drive?").
Yet, we are faced with the inheritance that comes with a browser in that simple stuff like "Back, Forward, Refresh etc". In light of this, the "NextRIA Generation" wants to look at changing gears and moving into another realm of possibility, that is desktop.
The desktop has a lot more attraction associated to it, but sadly once you breach the seal of the browser, you must now contend with security. Security is probably the hesitation of most, as once you go beyond the browser you are now kind of bound by the same rules most desktop applications, in that "Can this application be trusted".
This trust is quite important, as once you take an existing RIA concept and then pushed it into a desktop experience, you also suffer from similar inheritance the browser had (associated behavior), in that "Since I'm on the desktop now, will this technology be as good as my brethren desktop applications made with other languages (i.e. is it also secure)".
In order for RIA to succeed, it can't muddy the waters between "browser mode" and "desktop mode", as if one weakness is exposed in either context, both suffer simply due to hysteria and inheritance (Kind of like why Paris Hilton can't stay in her Grandfather's hotels when she's abroad - Brand sensitivity). Silverlight and WPF share the same language base (e.g.: C# & XAML ) yet both Runtimes are actually quite different. This is enough to separate the two from one another so that if Silverlight or WPF were to ever be compromised in terms of exposed flaws, then the other is unlikely to suffer the same consequence as we've made it clear both have different Language Runtimes.
Overall this is why i've placed my bets on Silverlight & WPF succeeding. I think they have the right ingredients and should you wish to move between the two, it's possible through commonalities between the languages adopted - yet - you're also insulated within two separate runtimes, thereby inheriting two levels of trust.
One is able to shoot for the Universal bullet, one that can sit in both worlds snuggly but provides elements of both in terms of depth within a persons hard drive. Yet, to do so means you compromise on trust trick then is to convince the world the compromise was worth it.