In the Leopard version of iCal, double-clicking an event shows a summary of the event, and to edit the name (or anything else), you must click the Edit button in the summary pop-up. To bypass the summary and edit pop-ups entirely, Option-double-click the event name. That selects the text for editing, and you can make any changes you want. Click outside the event to save your changes.

From Adam C. Engst