Actually it is you that hasn't been paying attention. John Kerry started this myth (lie) on Oct 8th 2003 in a debate with President Bush. His quote was "General Shinseki, the Army chief of staff, told him he was going to need several hundred thousand. And guess what? They retired General Shinseki for telling him that."
Although it is correct that Shinseki's advice was ignored, in fact his decision to retire was reported in the Washington Times nearly a year before his February 25, 2003 testimony.
In fact Shinseki didn't even recommend 300,000 troops. His quote in committee was "Something on the order of several hundred thousand soldiers are probably, you know, a figure that would be required," General Shinseki told members of the Senate Armed Services Committee today. General Shinseki made clear that he was providing only his personal assessment of postwar needs, and that the final decision would be made by the commander of American forces in the region, Gen. Tommy R. Franks. (Article by Bill Gertz and Rowan Scarborough, "Inside The Ring," Washington Times, April 19, 2002.)
I find it funny that some people simply assume that Generals who disagree are right, but the Generals that actually came up with the plan and executed the most successful military campaign in history are never given the credit for being correct.
Shinseki was right, Rumsfeld was wrong is the moronic chant. When the reality is Franks was right, Shinseki was wrong!