This is certainly true of NiCd and NiMH batteries. Lithium Ion, though, is better at keeping a charge:
Radio Shack Battery FAQ
"Storage Life Loses less than 0.1% per month."

Compare this to NiMH, which can lose up to 70% / month:
NiMH Battery Retention Curve

(edit) Actually, the 70% figure assumes storage at 40C (104F). It's only (!) 30% at "room temperature". The Radio Shack source says "4% per day" in storage, but doesn't give a temperature.

I'd trust a fully charged LiIon cell phone pack for a couple of months, anyway. Which is a lot longer than I'd want to be away from home. :-)