It appears that iOS developers can now download OS X 10.10 Yosemite without needing a separate OS X developer account. In the past, a Mac developer account has been required to get developer preview software from Apple, and will set you back $99, for a total of about $200 for developers with both iOS and Mac accounts.
One of Apple's stated goals at yesterday's unveiling of iOS 8 and OS X 10.10 was to get iOS devices and Macs to work together more seamlessly than ever before. By opening up Yosemite to iOS devs before they have to spend $99 to take a look, it's likely that Apple wants to encourage iOS developers to explore Yosemite, get to know it, and possibly begin writing apps for it. You will still need a Mac developer account if you want to publish your apps on the Mac App Store.
Developers can find the Yosemite preview in the iOS Dev Center. What do you think of Apple's move to open up the OS X 10.10 preview to iOS developers?
Thanks to Clint Brown for the tip!