The APPO seems to include other elements, like Zapatistas and indigenous groups (usually associated with PRI), as well as teachers and community groups.
Still, I'd say PRD is probably the most organized of the opposition groups, and probably is prominent in this opposition movement.
There was a serious incident last week when shots were fired -- apparently by the Governor's body guards -- at protesters who'd entered a hotel where the Gov. was holed up, and a prominent reporter (Ricardo Rocha of El Universal) was roughed up by...??? The APPO and PRD suggest the hooligans were military, given their discipline, youth and haircuts. There's a history of Mexican authorities using military units in civilian clothing to act as provacateurs during civil demonstations (justifying police intervention), so the story has some legs.
In other Mexican news, the teachers are still heading for Mexico City and the Fox Administration is looking to negotiate a settlement. I think the military fly-over was a "not so friendly reminder" of who gets to make the decisions... but I wouldn't be suprised to see Gov. Ramarez resign and an interim goveror from a minor party, or a "Mr. Clean" PRI figurehead to be appointed until the next election cycle. Maybe Dulce Maria Suares, who has a weird reputation as an honest PRI leader.
I had an email today from an artist friend in Oaxaca. The tourists aren't back, but everything's functioning despite the disturbances... and everyone is bending over backwards to make sure foreigners are unmolested.