I installed Leopard about 9 months ago using the DellEFI method. It has worked flawlessly since then, including waking from sleep (closed cover). Just recently, it stopped waking from sleep properly.
The screen will show-up like nothing is wrong, but the first clue something is wrong is that my home wireless connection cannot be found. Then items on the screen start to feeze (dock pops up, but won't go back, selected dock icon bounces then freezes) and then everything becomes frozen.
To start things up I need to do a hard reboot (hold the power button to shut down, the press it to reboot).
I am still running Leopard (10.5.7) and have hesitated to do further upgrades since it works so well. I can't think of anything that I did that would change sleep behavior.