I have spent a good deal of time figuring out how to power the Gumstix for portable use.  Initial tests with 3 AA (~4.5 V) batteries proved ineffective.  A standard set of alkaline batteries from Radioshack gave me about 1/2 hour of performance.  After that, the voltage diminished and fluctuated, causing the Gumstix to reset sporadically.  The same low voltage problems attended the use of rechargable (what type?) AA batteries.

I am now looking into several other power options.  Randell et al. were able to achieve 6 hours of continuous power using a 750mAh Lithium Polymer battery.  I am looking into the following battery/charger combinations.

It is a bit pricey, but should work.  I will update my findings on the Gumstix Resources page.