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.
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.