After lots of searching, I had given up, but there _IS_ a relatively cheap Aux / Ipod in solution for McIntosh units with the Clarion CE-NET port on the back...
One of these...
http://www.audio-discounters.com/aip-cl-id.html
It's made by these guys ( http://www.axxessinterface.com/ ) but I can't see this specific model on their site so I linked to a random retailer...
$55 USD roughly and it essentially emulates a CD changer and adds an iPod input (may not work with touch Gen 2 or iPhone 3G) an an Aux input.
I just had one installed today and it's great. Output level is a shade lower than the radio or CD, but it's a genuine Line In which makes me a very happy camper!
Cheers - Neil G