We have experimented with CodeMirror and several other solutions. In fact, Sage includes CodeMirror version 1 as the editor for DATA python files. In the single-cell server, we use CodeMirror 2. CodeMirror is a very lightweight version of the same sorts of ideas as ACE.
John is right. Not only have the options been too slow once you have lots of cells, but the options for syntax highlighting don't work perfectly across all browsers. Good ol' textareas do work across browsers. However, the last time we really tested switching the default cells was several years ago (before ACE and CodeMirror 2, for example). It would probably be good to try again and see if browsers and editor widgets are up to speed now. So: feel free to experiment with switching the cell types and reporting your results. I'd suggest trying CodeMirror 2 first, and if that doesn't work, trying ACE.
I think that people have tried to implement different syntax highlighting packages in the notebook, and they were all too slow.
This looks like it would be an excellent option: [ACE](http://ace.ajax.org/). I would love to have VIM keybindings when working in Sage. Looks like there are [native methods for transforming textareas](http://groups.google.com/group/ace-discuss/browse_thread/thread/01480dcd06c88efd).