Rael Dornfest (editor of the Google Hacks book, amongst other things!) mentioned a word on the blosxom mailing list which I have heard a times now, and which I think will become more popular;

mindwidth

A human’s capacity for thought and or mental processes such as attention. The amount of available mental “processing power” to perform a variety of tasks at any one time.

I like it 🙂 – Just for fun, ask Google who else is using ‘mindwidth’?