Looks like a 404 in about 80% of the requests I'm sending it. What's up with that?
If you'd like to see this phenomenon for yourself, go here:
http://api.msappspace.com/relay.proxy?opensocial_authtype=SIGNED&opensocial_token=MwrO1Q%2ftRYpGUyhGh8EYK1IvGAKg5Q%2f1te9KJ
You should see a Server Error. What you will see most of the time is a 404.