It’s just been pointed out to me the after much waiting, the BBC have started to make AAC streams available for their listen again content on the iPlayer. The bitrates vary from 96 kbps to 128 kbps from what I can see. This will possibly result in better quality. AAC 128kbps can certainly be an improvement over the same bit-rate MP3 but it does seem that the BBC have reduced the bandwidth and quality to AAC 96kbps to save us a whole 32 kbps !! Oh well, you at least still have the choice between MP3 and AAC. Make sure you have rtmpdump installed to be able to download AAC.
In get_iplayer, the default order of trying to download a Radio programme is: iphone,flashaac,flashaudio and realaudio. You can explicitly specify to use AAC mode by using
get_iplayer --type=radio --amode=flashaac [other search options]