It streams the audio via Safari (that's why you can't use Safari while its streaming). It's been possible to do that trick for a while. For example: listening to podcasts streamed off the BBC website in the background while using another app.
I wish Absolute Radio would tweak their app to do it too.
p.s I highly recommend the TuneIn Radio app.
For £1.19 you can record!, rewind, play in background, view track and show details etc.
Don't know if this will work, but the Internet shortcuts I use for Absolute Radio using TuneIn Radio are these:
http://tuneinmobile.com/background....o 105.8&url=http://mp3-vr-128.as34763.net:80/
http://tuneinmobile.com/background....io 105.8&url=http://mp3-vr-32.as34763.net:80/
You should be able to save the shortcut to your Home Page and then use them whenever you want to.
p.s I highly recommend the TuneIn Radio app.
For £1.19 you can record!, rewind, play in background, view track and show details etc.