I'm building a child theme based on the Sandbox/Blueprint framework. I've done this successfully very recently so I kind of know what's supposed to work.
When the child stylesheet only contains the header info and the import command for the parent's style.css, that worked fine. If I added any styling rules below the import command, all the styling went away. I use the Firebug Firefox plugin so I was able to see that absolutely none of the styling was being applied.
Well, while I was writing this, I figured it out. The lack of an ";" after the import command and "blammo!" the styling below goes away.
Live and learn.