Confusingly, both the CD based and DVD based systems say "Audi Navigation Plus" on the front panel. So how do you tell the difference between them?
If you have one large knob on the right surrounded by 4 shaped buttons and a screen that motors down to reveal a "CD" slot and 2 slots for SD cards - then you have a DVD based RNS-E and you must buy the DVD from an Audi dealer - or a seller on here or eBay.
If you have a large knob either side of a display that can only be angled slightly and a CD slot above - then you have a CD based RNS-D and you can buy the CD from TeleAtlas (my local Audi dealer couldn't get the CD's when I had a CD system, I had to tell them about TeleAtlas!) - or a seller here or eBay.
If you are SURE you have a CD based system then a DX CD from TeleAtlas with TMC enabled WILL work - I used the 2004 version in my previous S4 (which had an RNS-D) and had TMC working 2 years ago!
If you are SURE you have a DVD based system, then like the rest of us with an RNS-E you've just got to hope that the DVD promised in April 2006 WILL have TMC working in the UK.
If you're not sure what system you've got, then post a picture here and we'll tell you!