I think I've got to the bottom of the problem and I think it's at the beebs end and should be easy for them to sort out.
I've recreated the error in Firefox by spoofing the user agent details so iplayer thinks I'm using a ipod touch.
User Agent for FW 2.1.1 works:-
Mozilla/5.0 (iPod; U; CPU iPhone OS 2
11 like Mac OS X; en-us) AppleWebKit/525.18.1 (KHTML, like Gecko) Version/3.1.1 Mobile/5F138 Safari/525.20
User Agent for FW 2.2 gives the error everyone is getting:-
Mozilla/5.0 (iPod; U; CPU iPhone OS 2_2 like Mac OS X; en) AppleWebKit/525.18.1 (KHTML, like Gecko) Version/3.1.1 Mobile/5G77a Safari/525.20
Pictures available here:-
http://tinyurl.com/5qwvxt
Should be a simple case of the beeb adding the above details to the list of allowed devices that can use iplayer mobile, hopefully they'll do this quickly tomorrow when they come in (I'd be worried if they don't)