AFAIK, there are no currently available, mass produced hybrids using Lithium Ion batteries, nor are any of the models announced for the next year or so planning on using them. As of now, NiMH owns this market.
Search this site, I think the costs and energy density have been discussed, but then again, don't believe everything you read on the internet. I.e., I don't know how to separate claim from reality.
Lithium ion is used in the 58mpg Toyota Vitz hybrid, but it's only sold in Japan at the moment.
The Nissan Tino hybrid also uses a lithium-ion pack, and has been on sale in Japan since 2000.
Also, Hitachi have joined forces with some others to increase production of Li-Ion for hybrid vehicles, so it's a fairly safe bet they've already negotiated some deals with other Japanese auto manufacturers to supply them with LiIon.