they usually pride themselves on having them available the day they're announced (officially). but if its more than just a tiny speed bump it will sell out almost instantly and you will get estimated shipping dates from 2 weeks to 3 months.
for an intel PB- unless youre on store.apple.com 3 seconds after the release you wont get one for 3 months.
Considering that we're not supposed to see Intel machines until 2006, I'd assume that if there is any sort of PowerBook update in 2005 it will be a G4 still, though perhaps the last G4 PowerBook revision.