Hi Gernot,
It's seems that nowadays there is a lot of 'social platform gaming' for iPhone. I didn't know of crystal (but seems it's beta now, isn't it?).
Sometime ago, i get a message from 'Florian Heinle' from scoreloop, another 'social platform gaming', i don't know if now, scoreloop is more know or even better.
Maybe the only adventage of OpenFeint over the rest, is that its more know for the people (well, at least by now).
I have downloaded Atom to see AGON in action, it's seems as good as OpenFeint, at least in functionality (it has scores and awards which is the added value for any 'social gaming').
(I have added you as friend on Atom).
King Regards, fjsantos.