Lol, I did this for SC2 release.It wouldn't be my thought to take it off just for a keynote, but no judging here..since I take time off for my own geekery - video game releases.. (next one will be for the new WoW expansion....).
Well in the UK the keynote is at 6pm, which means many people will already be at home by then.
Take a day off? No chance.