I've just put live a change that adds a Trash folder into the Assets Library.
This works in the same way as the Trash folder for Content, though the Content and Assets trash are stored separately, and you can empty the Content and Assets trash folders independently of each other.
So when you delete an asset from your site, it's stored in Trash rather than disappearing completely. You are then able to Cut/Paste it back into a live assets folder if you want to.
You'll notice that all Assets that you've previously deleted are there in the Trash folder now - if you want to get rid of them permanently then head over to the Assets Library and click on the Empty Trash button and they'll be removed.
Please let us know how you get on!
Joe - Voice Admin