The Summer 2011 DC Comics event is called Flashpoint, and in it The Flash ends up stranded on a parallel Earth. Is this alternate Earth one of the 52 parallel Earths, as revealed back in the series 52, or is this meant to be a new Earth or replacement of an existing one?
NO. From the original announcement:
This isn’t a parallel Earth.
This isn’t a mirror world.
This is home.
After all is said & done, it may or may not be wrapped into the multiverse canon, except that DC is going to be getting rid, or at least diminishing the prominence, of the multiverse. Keep on eye out for Grant Morrison's long in the works "Multiversity"(famous for Dark Knight Returns crossing over with Watchmen in the promo art) which was said to "redefine the multiverse."