yep, I'm the original anonymous poster - bought Firecore since asking but before the replies, and of course found it doesn't work :)
it must be sending some generic user agent string.. I don't really have the tools for it at home right now (don't have a PC and wired network set up..), but if someone out there has, it might be worth a go at intercepting and rewriting the outgoing user agent string to the iPhone one using Fiddler to see if it works in principle..
(set Fiddler up as a proxy, share your wired internet connection over wlan, connect to that using the aTV, find out what the current user agent string is, then set a breakpoint on that, hit iPlayer with the aTV browser and paste in the iPhone one at the appropriate point - see if it works :)
alternatively get_iplayer might well run on the command line, but that's obviously a whole lot more fiddly than using the browser (but if it can be demonstrated to work, well that opens up the possibility of an app or firecore plugin in the relatively near future..)
hmm I also wonder if the browser's agent string is set in some config file in Firecore and hence might be user-editable - that bit I can have a bit of a poke around with at home..
(..if nobody's up for this I'll take my aTV in to work where I have all the tools for this ;)