Which iOS and Lion third-party apps support "Documents in the Cloud"?
I use SigleText (website | App Store link | MAS link) to share notes between my iPhone and my Mac; it uses iCloud storage.
As far as I know, iTunes Movie Trailer app and Doodle Jump app use iCloud storage to sync your settings between iOS devices.
Worth to mention: Instacast. The last version sync your podcasts (played status, favorites, resume playing location) thanks to iCloud. The developer said is going to launch an iPad version, so this feature should be very handy in the future.