Unlikely, much of Apple's current strategy seems to be in lock-in.

It seems developers can use APIs in both Mac and Pc. I see no reason why there will not be a third party client for iClouds in Windows.

I'll believe it when I see it. Just like there aren't any 3rd-party FaceTime clients, and I guess there won't be any iMessage clients either.

Currently all major players are competing on who manages to build the most closed ecosystem. I hope this is a phase that will pass.

They showed a Windows computer with pictures from iCloud during the keynote.