As long as all the components of the machine are structurally supported, and there's no risk of anything falling/collapsing, you should be fine. Just don't let any children or pets near it, or as Osgeld said, go sticking body parts in it. As long as you don't open the power supply (don't even THINK about it! No user serviceable parts inside!) its perfectly safe for you...just be careful you don't fry the machine.

For what its worth, I've run PC motherboards out of their case with PCI/AGP cards installed with nothing holding them except for the slot and they've been fine...just be extremely gentle with it and you'll be right.