He has released a statement on the matter.
"After careful consideration and many conversations with friends and family and the leadership of my party, I have decided not to seek the Republican nomination for Senate," Ridge said in a statement, adding later, "The 2010 race has significant implications for my party, and that required thoughtful reflection. All of the above made my decision a difficult and deeply personal conclusion to reach. ... To those who believe that the Republican Party is facing challenges; they are right. To those who believe the Democratic Party is without its own difficulties, they are wrong. No one party has a monopoly on all of the answers. ... And so my desire and intention is to help my party craft solutions that both sides of the aisle can embrace."
Not surprising, given that Ridge currently claims Maryland as his home state. Despite his strong Pennsylvania roots, that could have been used against him in both the primary and the general election.
I see this as an unhappy outcome for the GOP. I don’t see Pat Toomey as being able to beat Specter in the general election. Is there another strong Republican who can both win the nomination and the general election?