One of the biggest things I would make sure to cover up-front is that you are "renting" this space (you should work out some kind of specific terms), not acting as their on-site support and suggestion department for the product.
This is the sort of scenario that could work out really well for you, or could end up as the kind of situation that you end up dreading going into the office because you know you are going to get slammed by a ton of requests from them.
I would take time to spell out the terms of the "lease". EG: in exchange for 4 desks, access to the coffee pot and your own keys the client gets Y weeks of free service, future upgrades, etc.
Doing this purely as a hand-shake deal could lead to small or large problems later.
Other things to cover would be use of shared resources (copiers, conference rooms, etc.). Handling of guests (will their receptionist be aware of you and able to deal with guests, should you have any?), access to the office outside of normal hours. Specific area(s) that are "yours" for the term.