At that point, Apple should get rid of the "Pro" moniker since such a thin machine will most likely require sacrifices in power and expandability that will make it useless to anyone who has a compute-intensive workflow.
Even if Apple could fit a proper mobile GPU and sufficiently powerful CPU, the constraints of such a thin design will mean either clocking them down to a useless extent or having a laptop underside that routinely hits over 70ºC.
In addition, it would follow that Apple would adopt a MBA style of SSD. If that happens, the great ability to fill the MBP with 2.5" drives (or even just the ability to easily upgrade/replace one 2.5" drive) will be gone and the product will be virtually useless.
Now, before everyone goes "flame on" on my post, remember that I'm just speculating and assuming the worst. Thunderbolt has the potential to make a MBAir-style laptop useful, but the lack of available/affordable thunderbolt accessories is worrying and constantly having to carry external TB accessories just to recoup lost functionality is horribly backwards.
However, if Apple thinned down slightly, removed the optical bay and migrated to a (normal 2.5 "SATA) SSD+HDD setup then I'd call that progress. I mean, who uses optical drives anymore?