I'm hacking on a MySpace OpenSocial app at the moment and have wasted several hours trying to debug this: all REST requests to
http://api.myspace.com/ give me the error Authentication failed. Failed to resolve application URI "my oauth key here".
In the end the solution was to create a new app; the REST call succeeded immediately with the new app ID. Presumably some database migration failed on MySpace between me creating my first app (many months ago) and now, leaving my application unusable.