Yes, we're over three years away but the candidates are already starting to come forward. 2010 will be an interesting year here - an open Governor spot and a hot U.S. Senate race - so it's not odd that things are starting up this early.
The recent big news is that Bob Mellow, the State Senate Dem leader, is seriously considering a run but it's now looking like it's likely to happen -
http://www.postgazette.com/pg/07219/807566-100.stmHe's from the Scranton area. This guy is the definition of a weak candidate - he's old, has a bad temper and is only a State Senator (meaning very little name recognition even though he is the Dem leader). The GOP is very likely to win if he is nominated.
County Executive Cunningham is tricky. He comes from a very interesting area - the Lehigh Valley. That could be key in swinging the election to the Dems. I don't know much else about him though. This could be a tough race for the GOP if he is nominated.
County Executive Dan Onorato and Auditor General Jack Wagner are the candidates from out west. Onorato could be a strong candidate (a young guy) but Wagner is...blah. Onorato would give the GOP a good run. Wagner doesn't have much to him - former State Senator, 2002 Lt. Governor candidate (Casey's "running mate." He came in second) and now Auditor General. He'll be re-elected next year. He, like Mellow, is an older guy and not exciting at all. The GOP would probably be favored in a race against Wagner though his politics fit the state well.
Now for the Republicans - Attorney General Tom Corbett (who will face a tough re-election fight next year), former Congressman Pat Toomey, former Lt. Governor Bill Scranton and U.S. Attorney Pat Meehan.
Corbett - Might lose next year. He has a reputation for being quite the partisan.
Toomey - I don't see him doing it. I want/think a rematch with Specter is more likely.
Scranton - He's been staying involved so he's probably going to run. Not a strong candidate.
Meehan - Might run for Congress next year. If he does and gets elected, he won't run for Governor as a one term Congressman. If he doesn't run, he'll definetley look into a race for Governor in 2010. He'd be a strong candidate - conservative, law and order type of candidate from SE PA (appeals to swing voters down here yet satisfies the base).
There are a few others considering a run and they'd make things very interesting. Look for two hard fought primaries on both sides but the GOP primary will likely be a bigger deal.