I spotted that behavior in iPhoto 4 and reported it as a bug.

It turns out that it was a feature and I came to appreciate it. Before iPhoto 5, you couldn't delete a photo in Edit mode. So, when you were in Ediit mode and saw, say, that a photo that looked like a good thumbnail was really blurry when examined closely, you could double-click it and have that photo selected in Organize mode for easy deletion.

I've just been using the delete button in edit mode! iPhoto 4.