For the iPod event the other week it went down a couple of hours before the keynote started and came back up just after it finished. We've had similar downtime for most other keynotes in recent years.
The usual Tuesday updates usually go down about 11:30 GMT and come back up around 13:30. I know this because the new stuff always comes out just after my lunch break has finished.
But they added iTunes 9, could that make difference? Usually updates aren't on Sundays and don't take over 4 hours....