The development of a button-sized fuel cell by Cal Tech professor Sossina Haile has gotten a bit of attention (e.g., Treehugger, Gizmodo) due to its small size and relative power. The research was reported in last week's Nature (as usual, sub required for full article, but the supplemental figures include a nice illustration of the fuel cell structure). The power density of these fuel cells is quite high. Predictions that they'll eventually replace batteries in laptops and MP3 players are premature, at best. While the life of a fuel cell may be far greater than that of a battery, refilling a fuel cell means either inserting a disposable fuel cartridge or refilling the propane tank -- neither of which has the convenience of a plug.
The greater life and power density of a fuel does seem well-suited for microbots and microbot flyers, however, enabling a longer movement and sensor/signal life than batteries. As microbots are generally too small to recharge themselves with solar, long-life fuel cells have another clear advantage over batteries: easier refueling in the field.