I find putting a statement on the page to the effect "This item is temporarily out of stock. You can find similar items here." with a link to that item's category, or whatever is appropriate, works nicely. It provides the visitor an option and the page is not de-indexed.
I always recomment the same as Marshall. If you redirect visitors for a temporarily out of stock item: 1) to where are you going to redirect them? 2) where are you going to tell them the reason for the redirection?