We've already being using that for Electric Heli applications. Currently, the biggest problem with using Lipos for R/C applications are the fact that not many cells can handle the amp draw of current brushed motors. And the fact that Lipo cells are not as robust as nimhs or nicads and needs to be handled with great care.
I've been using a pack of Lipos for my brushless heli for months now and coming from 7 cell 3300 (7-9mins flight time) to a 2s4p pack (20 mins and counting), its certainly something worth investing in, but i paid something like 150US for mine...so i dun think its very practical for the R/C car scene yet.
....but imagine....7200mah pack + brushless...... pure bliss....