So I looked into the IDNA2008 situation a little more. (IDNA2008 being the latest standard for internationalized domain names, released in 2010, which happens to be backwards incompatible with the previous standard, IDNA2003, released in 2003.) As far as I can tell WebKit has no immediate plans to support it. Gecko does have bug 479520 which it appears to plan fixing in the immediate future. Whether they are going to take UTS #46 into account is somewhat unclear. Opera supports it since 11.60 but has faced some compatibility issues. No idea about Internet Explorer.
Quite the mess.