For those who are interested in analysing further information about this whole unladen/kerb weight subject. I looked back at some data from last year when during EVO's Fast Club article in May '06 the RS4 was weighed and so was the M6.
Now having used these very same scales in a recent group test for the magazine, I know how careful they are about getting accurate figures and also that the scales are calibrated correctly.
Both cars were specced with the options you would normall expect (they were weighed with full fuel tanks, so I have subtracted the weight of these from the actual figure);
RS4 - manuf quoted unladen weight - 1650 kg; Actual - 1694 kg (1740 kg with fuel)
M6 - manuf quoted unladen weight - 1657 kg; Actual - 1709 kg (1762 kg with fuel)
So as you can see the RS4 was only 15kg lighter than the M6. BMW's figures suggest the E92 M3 will be 135 kg lighter than the M6, so if that is true then the E92 M3 should be 120kg (or 264 lbs) lighter than the RS4.
The 997 C2S came in at 1460 kg (with fuel).