We discovered a great company a couple of months ago. You pay a small subscription per month (£12, about $17ish) and they'll send you 2 DVD's through the mail. Keep them as long as you want and then send them back in a pre paid envelope and they'll send you 2 more. We had 16 sent to us in the first month and as far as I'm concerned thats a bargain when compared to regular rental prices.
I heard about self destructing DVD's a few years ago, the idea as I remember it was the media layer oxidised and turned black. After a period of time the disc would become unreadable. That's fine in theory but give it 18 months and DVD-R drives will be down to $100 and the way will be open for mass copying on the scale of CD's. It isn't going to do any good at that point.
Gatso