From the OpenSocial reference section:

opensocial.DataRequest.newFetchPersonAppDataRequest(idSpec, keys)

It looks like you can grab data for a friend by id, but you would only be able to grab data saved by this application instance

Okay, a little more digging and I found these

newFetchGlobalAppDataRequest 

newFetchInstanceAppDataRequest 

 I'm assuming that Global allows you to share data between apps.  I couldn't find any documentation on it - other than a basic function description - so I'm hoping that is what you are looking for.  That's assuming, of course, that the App Data API is working .

Alright! Thanks, I've been needing something like this for a while. now only if the Data API would be a little less broken.. I suppose I could implement it for now in the hopes that they will get it running smoother in the future haha (we can only hope).

And I ran across global data before too, and thought I was saved, but then I read this forum post.

Chad Russell:

Hi all, we had to remove the ability to update global app data from the container for security reasons, sorry for any inconvenience!

 Oh well, but requesting the data by friend ID is all I really need for now.

 Yeah, I ended up rolling my own as well.  I've got registration and unregistration working (when user installs or uninstalls app, via the callback).  I've also got a name/value data store that is keyed by user and application.  You can read values for another user easily, by providing the correct ID and authentication information.

 nope, I gave up

I wrote my own app storage mechanism using webservices and a sql backend.  Works like a charm.