Thiswas mostly but not entirely unexpected.
Time to get out the speculation on successors.
My bet FWLittleIW is on Elijah Cummings being the front-runner. Good rep in Baltimore, likely to be popular in PG and Montgomery Counties, but how well can he draw beyond there? How would he play in Baltimore and Howard Counties? Past time for an African-American Senator from Maryland, if Illinois can elect two in the past 12 years.
Chris Van Hollen? Clearly in the market for a Senate seat, great candidate and very effective as a legislator but a little too new in Congress. (Full disclosure -- personal friend from 17 years ago) Waiting now should get him the nod when Mikulski retires in 2010.
Anyone else? Cattle call time.