After burying FolderShare for several years in MSFT development, and then giving us a crufty Office add on, MSFT seem to have sorted themselves out with Windows Live Mesh, which has recently come out of beta.

http://en.wikipedia.org/wiki/Windows_Live_Mesh

Note an important aspect of this service is that the data does not necessarily have to reside in the cloud (c.f. dropbox) but the service offers to merely sync it across your devices - for those concerned about people looking at your content "at rest" in the cloud it's quite sensible. There's a REST API for access to the service - perscon [*} sync service courtesy MSFT?

http://en.wikipedia.org/wiki/Windows_Live_Messenger_Connect

Right now in terms of apps Windows and Mac supported for syncing. Should imagine Windows for Mobile coming soon. No sync app for iPhone and IPad; web browsing gives "not available on mobile" so you need something like AtomicWebBrowser that can lie for you.

[*] This link previously http://perscon.net/, now points who knows where.