Credit and fiat currency will still be needed to support various social programs (gov't backed). Bitcoin will be a great additional currency to move goods around.
I guess it all comes down to how governments and banks react to declining deposits and tax flows. They can either freak out and enact all kinds of laws that force bitcoin underground (much like torrents and other peer-to-peer activities) or they can embrace it, yet try to enforce some measure of control, at least on the taxation end.
I am not very optimistic about them adopting the technology, however. The more likely scenario is full-on freakout mode before the inevitable death of their archaic systems.