I tested this in Leopard (10.5.8) but it should work with Snow Leopard (10.6.x). Please comment telling me if it does work in 10.6.x.
In Terminal, type the following
defaults write com.apple.dock mouse-over-hilte-stack -boolean YES;killall Dock;killall Terminal
Terminal will close, and your Dock will flash. Open a stack, and you now have a gradient when you mouse over an icon.
I know it’s old. Just thought I would post it here. Also, to reverse it, copy and paste the same code, but replace YES with NO.