I want to clarify this by saying THIS IS A RUMOR. But the blog The Vail Spot posted this about an hour ago that was picked up by Instapundit:

I’ve just heard from an impeccable source that Barbara Mikulski, the Democratic Senator who is up for reelection this November, will choose to retire. Mrs. Mikulski is expected to make her formal announcement in the next few days. Mrs. Mikulski seriously fractured her right ankle last fall just prior to Edward M. Kennedy’s death. Due to the severity of the fracture, she had to have open reduction surgery, that included the insertion of pins, as well as the use of special surgical boots, during recovery. She had tried to arrive in time for Mr. Kennedy’s funeral but was turned away.

This is the first I’ve heard of anything leaning towards a Mikulski retirement. I wouldn’t take it to the bank, but I wanted to bring it to the statewide conversation, because this would create a MAJOR change in the political environment and pretty much throw everything we know about statewide elections in 2010 out the window.