Next week, Tech Center Labs will release a NiMH battery pack Emergency Charger module for the Visor line of Handspring products. The module uses a Handspring Modem casing and can be used to recharge your Visor or extend battery life by 2-3 hours. The NiMH charger also recharges in the Springboard slot when charging the Visor from its cradle, car charger, or even 3rd party products such as solar panels.
The NiMH Emergency Charger will be available mid-March at a price <$50 and will include the rechargeable batteries.
Hopefully the company is hard at work on making a Treo Emergency Charger. I'm sure we'll see one eventually.