I have the exact same issue. Only for me, it's when I try to use a number in the name of the event.
Example, last night I took a ton of screen shots off my iPhone 4S. I sync'd them into iPhoto. And when I try to name the Event "iPhone 4S Screenshots for Resync" I get as far as "iPhone " (space included). As soon as I hit the "4" it ends my renaming session. If I type out "Four" it's fine, but the number itself, 4, or any number, seems to end the edit of the name.
Huge bug if you ask me. Since, well, People use numbers in their dates and names all the time.
And an update moments later. While I was trying to edit it, iPhoto was doing something over on the left. Once it finished, I then was able to do my edits just fine to the Event names.
OP - Have you tried letting iPhoto sit for a while to do whatever it is doing in the background and wait a few minutes before trying to edit the event names? And have you noticed if it was numbers vs letters that "stopped" the renaming session?