that’s the new shortcut to switch right between tabs in safari 1.3 that gets installed with mac os x 10.3.9. command shift [ being the shortcut to switch left. it used to be the left and right arrows instead of the curly braces. so all i can imagine is that they’re using command shift left/right arrow somewhere else that was conflicting in mac os x 10.4 tiger (that’s a mouthful). or they are trying to standardize the shortcut for switching between tabs in apps since so many apps are using them these days. presumably, they’ll changed the HIG to reflect this, if the latter is the case.