We've been using a Lithium Ferrous (LiFePO4) in the SV for a few months now (April '11). It's a 4 cell battery, and it only weighs about 1lb.
Since this bike is track only, it sits pretty much all during the work week, and hasn't had a hiccup yet. (I bought the charger just in case, but haven't needed it so far).
Link:
http://alienmotion.com/html/products...on_batter.html