When is the earliest that might we know if the Mariinsky and/or Bolshoi Ballet troupes will tour the USA in 2013?
Kev, thus far there are no concrete plans (of which I'm aware) for either company to tour the USA in 2013. The Bolshoi's big summer tour in '13 will be to London, at the ROH.
As for the Mariinsky possibly going to NYC or other US stop in summer 2013, the only rumblings have been about the Ratmansky Cinderella going to NYC (yawn...the same threadbare-scaffolding production that was in DC recently and in 2005).
My guess is that, should a Mariinsky Ballet appearance at the Lincoln Center Festival (or elsewhere in the US, in summer) come about, we will receive official word in the May 2013 timeframe...sometime after ABT-Met Season tickets have been sold and the ABT season well under way. Lincoln Center Festival is usually in the mid- to late-July timeframe.
Also, don't forget that summer 2013 will be the grand opening of the huge new Mariinsky II opera house, behind the old Mariinsky, in StP. I believe that all facets of the Mariinsky troupe (opera, ballet & symphony) will remain in their home town, in full-force, for the full White Nights season, except for a handful of appearances in Paris (Sacre du Printemps) and Salzburg in late May.
I heard the same thing directly from a company member that Mariinsky will tour NYC with Cinderella next year. But unlike engagements in other cities, MB always brings multiple programs to NYC, I hope it will be the same case for the upcoming tour if it comes to fruition.
Lincoln Center Festival usually announces the schedule around early to mid March, then pre-sale starts a week later, and general sale another week after.