The problem is that the iplayer site is serving up the high bitrate H.264 video as it would to a normal desktop. If you search the forums you'll see some discussion on things like get_iplayer to get the mobile versions of streams.

We might very well be able to do something with spoofing a user agent to get to the mobile version of the site (unless flash 10 brings us a better 264 codec and makes it unnesecarry)