Macbook Pro as extension display for iMac (both TB equipped)

I would like to use my macbook pro as an extension monitor for my new iMac 27" 3.4ghx i7. Both are thunderbolt eqipped. I can use the iMac as an extension display for the MacBook pro but cannot find a way to swap their roles.

It would be even better to be able to access the hard drives etc on the macbook pro as well but for now I'd be happy with just the extra display real estate!

Thanks

Don