According to this thread at the MyDellMini Forums, the just-released Mac OS 10.5.7 update breaks hackintoshed machines like the Dell Mini 9/Vostro A90 netbook.
This shouldn't come as too much of a surprise and most self-respecting hackintoshers know better than to install any old update on their netbook willy nilly.
If you do arbitrarily install 10.5.7 on your netbook you'll be in for all kinds of trouble including garbled screens, shutdown troubles, beach balls and no finder.
It seems that there's a fix if you boot in safe mode and reinstall the latest DellEFI but you'd be wise to wait a couple of days until the dust settles before installing the latest OS X iteration on your hackintosh.
Tip: Tycho Sean