Well, I'm on their mailing list and there appears to be a fair amount of debate about this. Some want it to be written in C, others say C++, one person mentioned Objective C, there appears to be a fair amount of support for development in a meta-language (oh yeah, that's good: people who want to help have to learn a new language) that gets compiled to C or C++ and others are advocating a "MicroPerl". The latter involves writing Perl 6 in a subset of Perl 5 that will get compiled down to C and optimized by hand where necessary.

All in all, I've seen some really intelligent ideas and some grotesquely stupid ones, but there doesn't seem to be a consensus yet (unless I missed a few e-mails).