Nevermind, i have found a nice example on goolge code http://code.google.com/apis/opensocial/docs/0.8/devguide.html#Persistent , that shows how to store and retrieve the data, and i tested the code and it worked fully :)

Is there a size limit on the value param in calls to newUpdatePersonAppDataRequest? The parameters I store may vary in name, so easiest is for me to store a JSON string of name/value pairs with a single "prefs" key, but I imagine I'll hit a size limitation at some point. Dont' see it...