The OLPC Learning Club DC repeated the experiment of running an XO-1.75 on a solar panel, with no battery involved. They tried this on a cloudy late afternoon, rather than a sunny early afternoon, and had to switch to a larger panel, but were able to make it work for some 20 minutes.
I look forward to further creative power tests and batteryless setups! And to any additional power-saving tweaks. (these tests have been run with suspend off. another interesting stat would be howl ong it takes to charge the battery while actively using the XO with suspend on.)