Its nickel which is present in the alloys they use. Unfortunately once you're sensitised to it then you're screwed. I am unfortunately screwed.

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.

People are allergic to nickel, which is used in aluminium alloys.

Humm

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)

It's possible. You can nickel treat alu to give it a consistent durable finish as alu oxide is quite easy to dent and this deposits some on the surface.

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.