Vacation housing swaps are popular. I'm thinking about doing it for my place. Is it common practice to have a contract between the swappers? Or is it something that is on-your-honor?
Good thinking - you've never met these people, and they may have all the good feedback in the world, but you want security and the peace of mind that your place is covered.
There are some sample contracts online you could copy - such as this.
If you're feeling uncertain about asking them for it or how to go about arranging it, eHow has a nice article on how to negotiate a house swap contract.