The President used over 20 different pens to sign the health care legislation.
The rationale is fairly simple. The pen used to sign historic legislation itself becomes a historical artifact. The more pens a President uses, the more thank-you gifts he can offer to those who helped create that piece of history. The White House often engraves the pens, which are then given as keepsakes to key proponents or supporters of the newly signed legislation. When Lyndon Johnson signed the Civil Rights Act in 1964, he reportedly used more than 75 pens...Personally I think all of this is rather silly. I suppose I could offer a rant about the waste of taxpayer money for the pens and for the time/salary of the lady depicted in the video, but I suppose there are more important things to save my outrage for. Still, kudos to GWB for taking a more sensible approach.
Not every President goes for the multipen signature, however. President George W. Bush preferred signing bills with only one pen and then offering several unused "gift" pens as souvenirs. Even a piece of legislation as famous as the Homeland Security Act got only one line of ink. When it was over, the President is rumored to have pocketed it.