Arlen Specter to convert to Democratic Party!

Wow! CNN just reported that Arlen Specter will join the Democrats (no link yet, but hopefully this story will flesh out soon). This is huge news. He will most likely remain fairly conservative, but what a smack in the face of the Republican party. Also, considering he is up for reelection in 2010, do you think this move will help or hurt his reelection chances? Will any Democrats run against him in the primary?