Personally, it's difficult to imagine a web-based IDE having much use; how would I compile or debug anything?
There's a microcontroller dev board called <a href="http://mbed.org/>mbed</a>; which would be a really nice little Cortex-M3 board, but it is only programmable through their weird <a href="http://mbed.org/handbook/mbed-Compiler>online web service compiler thing</a>. I admire the mbed folks' creativity, but that takes vendor lock-in to a new level, and it's an absolute deal-killer for me.
But, to get this right, you want to use one because a) you can't sit and concentrate and b) your device of choice doesn't leave you no other (simple) choice but using something web-based. Fixing one of those premises might be a lot easier than creating a good browser-based IDE.
The key to having a good web based UI is the ability to debug, compile, test. If you can set that up, you may be on to something.