No, though it may work that way initially. Apple has made APIs available that allows any developer to integrate thier apps with the iCloud service no matter what the platform.
When it rolls out this Autumn it will "just work" with the core iLife apps and OS X Lion, and if it does "just work" as promised I'd wager a lot of developers on the fence about integrating will hop off it (and add it to their applications) fairly quickly.