As a win32 developer moving into web deveopment the last few years, I found the
web desktops based on extjs very interesting.
Coolite Desktop (broken)
Ex
Like others have said, this has been around for a while. I think if it was gonna take off, it would have already. I don't think it's a matter of technology catching up, I think it's a matter of developers not wanting to put time into a tried-and-didn't-take-off technology.
Is the desktop a good way to organize your apps? Most people I know don't use their desktop very often, it's rarely actually visible. The Start menu in Windows sees much more use and it's analagous to a list of browser bookmarks.
Also, my opinion is that Silverlight will allow better browser-based user experiences with less development time.