I think that A123 is coming out with a pack that is the same size as a 6 cell NiMh. Look at page 92 of the January 2008 issue of RCCA and it shows a pack that looks flat and has 2200mAH.
We spoke about that and one member here tested with them, but no word yet on availability. I also saw them being used in a Tamiya, that's when I noticed the pack was different from the normal packs they sell now.