Following the link and reading the FactCheck.org post on the Biden-Palin VP Debate
... it looks like that article Corey posted is right. They never corrected Biden on his citing of the wrong parts of the constitution, amongst many other things. They do make corrections to their "checking" as can be seen on their homepage, maybe we will see that soon?
It doesn't really seem as though FactCheck.org is biased, as they knock both Republicans and Democrats, but they do seem to have missed some key points.
If their job is to check facts, they should probably check for all of them, right?
Usually, it helps to read several news sources in order to get a more true picture of what's going on. I visit FOX News, CNN.com, FactCheck.org, BBC.com, and read several other newspapers and news sources a day.
Don't rely on one place or you won't be getting the whole story.