I was very tempted to go the VAG or Dension route, but decided to go the budget route instead.

I bought an "AUDI / VW / SEAT / SKODA Aux Adapter - iPod, MP3 - MFD" from ebay (eg ebay# 230173379995), which plugs into the CD-changer socket on the back of the OEM headunit at one end and has a 3.5mm stereo plug at the end to plug into the headphone socket of the mp3 player.

Not quite what I wanted, but a trip to Maplin resulted in a 3.5mm chassis mount socket which I used to re-terminate the iPod end and installed in one of the dash blanks.

OD, it doesn't recharge as you drive, and you have to set the volume on the iPod as well as the stereo, but I can do the Octavia and the Fabia for about £65 rather than £200. And there's no compatability issues swapping between the missus' video thing or my shuffle or any other mp3/walkman for that matter.

Nice idea mate.