If you view other illegal stuff online then it being saved to your hard drive temporarily is enough. When the file gets deleted it still remains on your hard drive and that can be recovered. It doesn't matter how long you had the file.
I'm not saying that watching a single movie is going to get you sued, but if you are doing enough to get that sort of attention then it can be held against you.
The internet isn't at fault for making you download things, you can download plenty of things legally!
Why not just zero out the data? I actually have a nice little widget that does that.
Also, if you put more stuff on top of that data it will be over-written.
Also, dosent Leopard overwrite the data it deleted a few times? Its "Secure Empty Trash" I think.
One more thing, what is the likelihood of your HD being randomly searched by police anyway?