Yes this concept will work, I know or organsiations who are starting to adopt it. It only really works with floating pools, where each image is identical. You can simply set your floating pools to use all the local datastores then allow it to rebalance across those pools. Be careful about overcommittment levels though as you will have limited storage.
The other conept is to use SSD as a write cacheI havent looked at vsphere 5 with SSD drives yet, but on the datastore there is a column called something like SSD enabled? If you work out how to enable that, I believe it will automatically use SSD as a write cache before hitting your SAN.
Two separate concepts, which you're probably better off using separately as you wouldnt use an SSD write cache if just using the same local SSD drives anyway.
I was speaking to the creator of Xen a while ago, and his view was that the industry will gradually move away from the concept of a large shared storage array, and end up using cheap local storage in this manner for services which dont require 99.999% uptime, like virtual desktops. I tend to agree with him. This is why Xen and Vmware have introduced local caching technologies and storage appliances (Xen to follow on the appliance, I suspect)
