Thought I might share this as even though it's still a lot of money, it seems a great option compared to a RNS-510.
While I was in my local VW dealer earlier, picking my GTi up after having a new Cambelt and a service, I noticed a really smart Kenwood Multimedia Nav unit on display.
It was a DNX520VBT and is shaped the same as the OEM headunits.
Check it out here:
http://www.kenwood-electronics.co.uk/products/car/vw/DNX520VBT/Specs here:
http://www.kenwood-electronics.co.uk/car/vw/dnx520vbt_spec/It was £942.13 inc VAT for the unit, £995 inc fully fitted - if you wanted the iPod/iPhone cable this was £35 extra.
The guys were really helpful, gave me the leaflet and a print out about it and we had a good chat - they reckon it's better than the RNS-510 - the built in Garmin satnav Bluetooth and iPhone/USB connectivity with full control being the major benefits. It even works with the steering wheel controls!
Having played with my mate's RNS-510 I can confirm this Kenwood is probably just as good if not better.
There are a couple of things though - as far as I can tell the iPhone/iPod connector has to go in the front USB and aux sockets, which doesn't look great, and there was no mention of integration with the Computer display on the dashboard.