There's no word on a fix, other than signing out of the PlayStation Network while you're playing. Obviously, this will prevent you from playing online. PlayStation Forum moderators are looking into the issue as a matter of urgency and hopefully we'll see a solution soon. We've experienced the freezing ourselves (and yes, our friends list is massive, thanks for asking), but an official cause hasn't been confirmed by anything other than anecdotal evidence at this stage.
[Thanks to everyone who sent this in]