updated 01:01 pm EDT, Thu July 24, 2014
Accessible only to Beta Program participants
As promised earlier this week, Apple has begun seeding the first public beta of OS X Yosemite. Access is limited to the first million people entered into the OS X Beta Program. That requires having both an Apple ID and a Mac running OS X Mavericks.
Apple cautions that the Yosemite beta shouldn't be installed on a primary machine, since it's still in unfinished shape and not fully stable. The final version of Yosemite is due sometime this fall, possibly in late October alongside new Macs. Apple often prefers to launch OS X updates in parallel with new hardware.