An expression meaning "going past the point where you can go back safely". After "crossing the Rubicon," one can't go back. The expression comes from Caesar (ie Gaius Julius Caesar of the romans) as he crossed the Rubicon river with his army. Julius was being threatened by Pompey, then in charge of Rome, and so decided instead to march in with his army, starting a civil war. Had he returned peacefully, he might have been sentenced to death, and risked that and the lives of many men if he attacked.
Special thanks to QXZ, and
http://www.geocities.com/Athens/Acropolis/2100/caesar/civil.html