ultracapacitor use in hybrids

anyone have any take on why current hybrids haven't made use of ultracaps yet? the closest thing i've seen to-date is a bmw x-5 hybrid prototype. the ultracaps only allowed it to have a 7-second boost, but my understanding is that ultracaps do a better job of collecting waste energy from braking. here's the article: http://www.autoexpress.co.uk/?http://www.a...ry.php?id=38421

any thoughts on this? it seems that oems could use ultracaps and a battery to provide a quick boost at first, followed by steady power for lower speeds.

on a related note, why is it that we aren't seeing lithium ions in hybrids? cost? availability? are there disadvantages to li ion?