http://wiki.sugarlabs.org/go/Nomodeset
The issue here is that some new software/drivers for Linux which do something called 
"kernel mode setting" for graphics cards haven't been sufficiently tested/developed 
for the particular cards with which you are having problems. Specifying 'nomodeset' 
when the system boots basically tells the system to use an older set of software/drivers 
which does work (but may be slower or have some other issues that people would like to eliminate).