Scientists at MIT have discovered a way to perform water electrolysis efficiently enough to do it with power from a photovoltaic cell, effectively eliminating the need for chemical battery storage of solar power. 

http://web.mit.edu/newsoffice/2008/oxygen-0731.html

Short (25 second) video:
http://video.google.com/videoplay?docid=-1528057721968624553
A balloon with a solar collector.