First Solar powered airplane took flight on Dec 5, 2009 in Switzerland. Though trip was short one and didn't run at high altitude but definitely big step towards solar power aeroplane
Solar impulse is the company running this project and has long term objective to run for longer duration. The next step of the project is set for Summer 2010 when they will attempt to run 36 hour non stop flight. Later they have plan for round the world flight which would need 25 days.
I personally felt this looks very promising and they may not able to commercial in short span. However such experiment show that solar plane is possible and can be achievable with further research and optimization.
Source : www.solarimpluse.com