Picked up the Marantz MCR603 last week, all hooked up to the router and assured by the unit that my firmware was up to date, have successfuly upgraded/activated the airplay, and now the unit has signalled firmware update is available.
So, running through the usual process I seem to be stuck in the infinite loop, as 'athacker' above. Culminating in the downloading/updating 13/14/13/14mins then 'Connection Failed:A4' (or perhaps AE).
Oddly enough, I am seeing the "M-CR603 Upgrade" displayed briefly (after the "Hello" display) as chebby points out. But after a good power down/up airplay is still not working (although listed as active and upgraded in the 603 settings) and the firmware is still supposedly out of date.
I can live without the airplay for now, as seeing as the unit says that's all okay, I can only assume its a networking issue at my place, although IP's are all assigned correctly etc. So I'll go back to checking the Mac running Lion.
The infuriating thing is the constant failed updates when the connection is demonstratably working (through internet radio at least).
The quest continues at the moment. Will be sure to update!