Jonny Ive is crying right now that his brushed aluminum is being cover up by vinyl.
This is a fairly big issue for a lot of people. Even my doctor asked if I use apple products when I went to him with the rash as he's seen it a lot.
I find it rediculous that they would even consider using nickel based alloys.
Maybe it's this one? http://en.wikipedia.org/wiki/6061_aluminium_alloy (a guess from here: https://discussions.apple.com/message/8612485)
Anyway, when I'm using a wristwatch I don't have a problem with the case, but the strap is often itchy (not to the point of irritating the skin, though)
Another thing I've seen is that a friend of mine's MBP has started to dissolve. It is pitted and looks burned. Turns out some people's sweat eats them as well.
Probably a bad material choice if you ask me.