I am not certain that Documents in the Cloud is coming to the PC & Mac. I would hope it was, but on the iCloud site it seems to purposefully tip-toe around the issue. On the "What is iCloud" page they have an animated graphic displaying how syncing works. It shows photostream, music, apps and books syncing between an iPhone, iPad and a Mac. However, when it comes to Documents the graphic changes and the Mac disappears, showing documents syncing between a iPhone, iPad and iPod Touch.
Also the Documents in the Cloud page makes no mention of Mac or PC and only shows iOS devices.
Seems like Apple is trying to tell us something by not telling us something. Hopefully, they just haven't figured it out. However, it maybe more difficult since I believe Pages documents on an iPhone aren't stored as .pages files like on a Mac, but as some iOS specific document type, since you have to export them to Pages. Perhaps this complicates syncing since all files would have to be constantly imported and exported for the process.
Plus, iOS lacks many fonts used on desktop machines. Synced documents would suddenly look very different on the iOS than on the Mac. For people in offices where certain fonts are required, this could cause issues.