Does anyone have any idea which older Macs will be capable of running Yosemite?
Pretty sure Apple does.
But seriously, any Mac capable of running Mavericks is also capable of running Yosemite.
Here's the full list: OS X Mavericks and Yosemite Compatible Macs and System Requirements @ EveryMac.com
As someone who had been testing Yosemite for some time, I would add to this list that you really should be running eight gigs of RAM or better. Those machines that are limited to less than 8GB of RAM are going to have an inferior experience than those that can run 8GB or more.