Stephen O'Grady has an interesting Q&A on SaaS which addresses the 4 Runtimes...
Q: So if I understand your argument correctly, you’re saying that these frameworks are pointless given the increasing popularity of SaaS type applications? That the writing’s on the wall, and it’s the browser?
A: No. There is now, and there will be for the foreseeable future, a need for rich client applications. Browser based applications have done a marvelous job over the past few years addressing some of their less forgivable shortcomings: interactivity (Ajax), featureset (plugins), and most recently offline access (Dojo Storage/Flash, Slingshot, Zimbra, Firefox 3.0, etc). But I’m not using one to write this post (that’s in Scribes), or handle my running IM sessions (those are in GAIM). Even should every conceivable technical limitation be remedied, however, there’s still user comfort levels and preferences. Rich clients, in other words, aren’t going anywhere.
"Grand Desktop Ambitions: the Q&A" | tecosystems (6 Apr 2007)