seems like a great idea. toolchain support may be an issue for a while, especially if you link against libraries with a different ABI. one thing to keep in mind is that the performance gain from e.g. more registers may not be as big as you'd think, since the hardware already plays lots of tricks to get around i386's limitations (e.g. register renaming) and likely extracts a good bit of the available ILP.
Actually main line kernel support is one of the last things that's been needed to get this going. gcc 4.7, and the latest glibc and binutils have had support for a little while. Now it'll still be a while before distros start supporting it as a first class citizen but the pieces are now there.
