I was looking at some lightweight Laptops and I was having a peek at the Toshiba R600 when I noticed that they offer OpenSolaris as an option instead of windows if you want it.
Considering the Portage R600 weights under 1KG has a DVD burner and a proper 12 inch screen with an Intel Core2 Duo processor thats the weight of an ultraportable, with the functionality of a proper laptop and with OpenSolaris installed you should get the speed of a decent PC.
laptops.toshiba.com/laptops/portege/R600
www.opensolaris.com/toshibanotebook/Plus you can also get a version with a 512GB SSD drive