I can see this being useful for introductory programming classes, but not for real projects. For a real programming project, there are a lot of things you need which are very hard to implement in a browser: a debugger, profiler, version control, and code completion, to start. And then there's the whole issue of the host running people's arbitrary in-development code on their server, which may accidentally spin into an infinite loop, or deliberately send spam, exploit local root exploits on the host system and so on.
The browser is good for many things, but it is not and will not ever be a good IDE.
A great place to start with these IDEs would be programming courses in the learn-to-think-algorithmically genre -- those often taken by non-majors. The overhead, in terms of both set-up time and learning difficulty, is a significant burden for a single semester course and often detracts from more meaningful learning.
Phone interviews were you ask someone to write some code to demonstrate their skills would benefit tremendously from web-based IDE. Makes it easier to filter out not-so-great applicants earlier in the process. Might give you a better idea to whether or not a candidate is worth bringing in for a face-to-face interview.
Great idea! That means I'm not tied to one computer and I get to use the same IDE everywhere with my personal settings. One thing that bugs me the most is when I have to copy my custom settings over to other computers. I think web-based IDE will solve that problem.
for several IDEs the settings can be exported or are even stored in a file that can be synched via subversion or some other revision control. I keep all of my emacs customization files in subversion and can quickly replicate my environment anywhere.
As for the suggestion that web IDEs are becoming mainstream, I say: far from it. Most developers haven't even heard of bespin or similar projects. Personally, I'd be surprised if they ever go mainstream.
Plus developers generally have pretty wonky setups that are crazy different from each other. Ever tried doing paired programming at a small dev shop? I couldn't type on anyone's keyboard, everyone had their own key mappings, keyboard layouts, shortcuts etc. Some had shorthand hostnames in their .ssh/config, non-standard terminal settings, crazy scripts in their .bashrc files, different virtual machines for different environments, etc. I'd say it would be impossible to convince a developer that this is the only IDE they can/should use.
In situations like this, it's pretty nice to just put your ~/.vim folder up on some version control and viola, your .vimrc, themes, and plugins all in one place.
The browser is good for many things, but it is not and will not ever be a good IDE.