To be "fair" to Roberts, he made his remarks at the morning press "Gaggle", not at the televised press conference. The gaggle is where the reporters ask questions and McClellan answers the easy ones and takes the rest to get answers -- which he will then give when the reporters publicly ask the same questions at the press conference.
If not for this procedure, half the questions at the conference would end up with "I'll have to check", since the press secretary is the spokesperson for the white house and can't answer unless he knows what the white house's answer would be.
If Roberts had asked this question at the press conference, I think it would be right to make a bigger deal out of it. But given the circumstances I think the apology was sufficient.
Oh, thanks, I had no idea that was how it worked. I don't like to watch the press conferences anymore.
I still think he owes more than just an apology, tho.