In iCal 5 (under OSX Lion) you can move multiple events forward or back in the calendar at the same time.
Multi-select the events to move by Shift-Clicking each event. Then, while holding down both the Control and Option keys, press the keyboard arrow keys to move the selected events.
In day or week view up/down keys move the events forward/back by 15 minutes, left/right keys move forward back by a day. In month view up/down move the events forward/back by a week, left/right still move them forward and back by a day. There seems to be no effect in year view.
Note: This behaviour is new to iCal in Lion and will not work in iCal for OSX 10.6 or before.