For some time I have watched the XUL effort at Mozilla. It is interesting as more and more people talk about the creation of "rich apps" (network centric apps.. whatever) to watch various approaches rise to the top. The strong efforts by the eclipse and netbeans camps to form their solutions about their respective architectures is great. There is also the efforts by M$ and their XAML goals eventually. I don't really think XAML is all that "Internet" when you have to run it on a windows machine though.
However, I have been interested to see that XUL, which I felt was only going to be a solution for Mozilla's own apps like firefox, thunderbird and sunbird, gain some traction in the community. The Songbird (http://www.songbird.org) effort is likely the one with the highest visibility, but Democracy TV (http://www.getdemocracy.com/) is also popular. I recently saw on a on-line episide of Command N where they talked about Cletx (http://www.celtx.com) which I guess is also based on XUL. They say it's a firefox app, but I assume the meant XUL.
It would be nice if we had a true XML UI language (yes, perhaps XML is a bad method to represent UI's ... that is another topic) that was a standard. Not that M$ would bother to support it but XUL can't claim to be a standard yet for anything but Mozilla apps.
So I don't know.. if you want to make a true Internet application I guess there is not set of standards to work with other than AJAX approaches.