All of the aftermarket ones mentioned so far re-transmit the DAB audio on an FM channel so that you can hear it through the existing radio. I've read that this can reduce the quality a bit (but have not heard it for myself).
Dension have an add-on DAB module
for the Gateway 500S media controller (which can control an iPod or USB stick for those of us who don't have that OEM option on their iDrive's). It's only available for the CCC-type system at the moment, the CIC one should be available early next year.
I also want to investigate the possibility of utilising the OEM DAB antenna. I haven't discovered yet whether my E91 already has the antenna in the rear window - if so, I would just need a replacement diversity aerial splitter.