[argarg]

Unfortunately OSX uses a badly outdated openssh version just like many other unix tools and it doesn't support the -W option. You could try upgrading openssh using homebrew if you want.

[carlesfe]

I'm using Ubuntu 10.04 and it doesn't have the -W flag either.

[argarg]

You need openssh 5.4+. I am running 12.10 and I've got 6.0p1