I've switched to a laptop with a logitech lap desk (http://www.amazon.com/Logitech-Comfort-Lapdesk-white-grey/dp/B001NJ0WCS). Seems to be working well enough for now, although having the comp in my lap presents other ergo issues (slouched shoulders, etc.).

I'm highly tempted by a balance ball as oppose to a chair (assuming I go back to a desk), but see my previous comment re. my son. A giant ball in my office? That's just asking for havoc. :)