President Felipe Calderón openly rejected the possibility of PAN backing a PRD candidate for 2012, such as Marcelo Ebrard. Marcelo Ebrard essentially says the same thing - that an alliance with PAN is "improbable." Yet Calderón pointedly left the door open for a "citizen candidate," or a person who doesn't belong to either - or any - party.
Two names that frequently appear in this regard are Juan Ramón de la Fuente, and to a lesser extent José Woldenberg - former UNAM and IFE president, respectively.
The PRD also notably contains what one may the "plague on both your houses" group, or a sector that rejects both Marcelo Ebrard and Andrés Manuel López Obrador above all in the ADN party faction. Should things get too nasty in the run-up o the election, this group might gain momentum. And their candidate could be, the aforementioned de la Fuente and Woldenberg.
All speculation, of course, but that is all the rage within all camps in these days of declinations and declarations.