Brighthand has a review of the Palm Power to Go, an external battery pack for Universal Connector-equipped Palm handhelds. At US$100/C$150, this clearly isn’t for everyone — not when car chargers and AC adapters are so much cheaper. Palm-branded accessories are usually pricey in any event.
But it sounds like a good idea in certain, limited circumstances. I can think of two: where you are using it heavily for prolonged periods, and it’s inconvenient to recharge it by other means (i.e., you’re using it with the battery pack clipped on), or you’re in a location where charging by other means is simply impossible — say, out in the field somewhere. Ed’s review has some other scenarios.