Some charging/power accessories are good for both cell phones and PDAs. If you're only packing a PDA and cell phone, but no notebook, there are some two-in-one options, like Belkin's AC/DC Converter, which appears to be an AC adapter to your phone's and PDA's car-charger cord. Nifty!
For plug-in flexibility, Instant Power (formerly Electric Fuel Corp.) offers a 3-in-1 charger ($29.95), that can plug into car or USB. For recharging while away from electron outlets, there's Instant Power's "packet of electrons"a disposable 3300 milli-Amp-hours (mAh), air-activated zinc-air foil-pakthat lets you simultaneously work and charge your PDA or cell phone.
For 700900 mAh rechargeable batteries, you can expect to get three charges. An unopened battery packet is supposed to stay good for about two years.
Instant Power also offers a variety of individual options and two-way kits, available for less money.
My opinion: If you think one of these may be right for you, be sure to try it before you count on it.