I wouldn't count on a Tau codex any time soon.
Blasphemy!
From what I'm hearing about the rumors for codexes this year (which as of right now is on par with DE and GK) we're supposed to start hearing about a Tau codex in quarter 4 of this year. As I heard it it was supposed to go DE, GK, Necrons, Tau and then some tenuous rumors of Eldar. But, who knows how true they are.
It does seem kinda strange that the super elite and rare grey knights will be fielded en masse when their new codex comes out. I guess we'll have to see how they handle it.