Weber web claims that its heads is made with 17-4PH grade steel. I assume they are talking about current heads, not DLC&ARC covered ones. I have seen this paper about that kind of steel:
http://www.aksteel.com/pdf/markets_products/stainless/precipitation/17-4_PH_Data_Sheet.pdf
and it seems a good quality, not rusting danger steel. But we don't know if old heads are made of the same grade of steel or what. And rust problems with DLC&ARC heads are not rumours, here you can see a very graphic example:
http://www.afeitadoclasico.com/foro/feedback/weber-arc-malus-t13195.html
I must clarify: for me Weber ARC is a fantastic razor, probably the smoothest one I have tested, but I think that is a honest point warming people with any issue about quality/materials no matter how good it shaves.
Cheers!