Actually... I updated to 0.12 and at the same time switched from ghc 7.6.3 to 7.10.3, and found the same thing. I had to move the "args <- io getArgs" directly into "main" before invocation of the "xmonad" function.

I haven't yet looked at the code but this seems surprising. I also suspect it's not related to a change in xmonad, since the only core changes (and only a core change could have any effect here) were documentation.
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