*HUGE SIGH OF RELIEF*
I checked the power meter, and it read 0%. The backup battery was still reading normal, so I had at least 30 minutes of reserve power left, but it was still a pretty close call.
I need to decide between buying an extended battery, would would give me double the battery life (over 180 minutes of constant wireless usage), but make the PDA heavier and a 1/2 inch thicker, or to buy a 3-in-one power adaptor, which would let me recharge the PDA anywhere, IF I actually had the cable with me. They'll cost about the same.