Can I create a MatrixStore cluster using MacBook Air as a platform?

Theoretically I can see no reason as to why you could not. The lack of ethernet ports will make it a bit harder but it is likely you will find a way as only a complete nutjob would suggest such a scenario ;-)

It would be interesting as all internal and external traffic would go over the wireless network meaning that each node (a macbook air being a node) could be located around your office or house. As long as they were in range of the wireless hub you would be sorted..

One to try out for fun ... not the best $/GB archive solution you will ever find ;-)