When I see a photo I want to edit, I like being able to double click on it and have iPhoto handle the editing. iPhoto's default is to "in-house" edit; double clicking does what you want it to do 90% of the time. However that 10%, when you want to move it to Photoshop, you are left with a few inelegant options. Conversely, if you set the default to open in Adobe's app, then most of the time you double click on a picture you are forced to retreat out of Photoshop and use the lower "Edit" icon in iPhoto.
While right-clicking on a photo in iPhoto the other day, I noticed a grayed-out option for opening the photo in an External Editor. I assumed that the option was grayed-out because iPhoto just didn't know what external editor I had or wanted to open the photo with. I scoured the menus and preferences, but found no way to set the external editor without just changing the default behavior of a double click, and thus annoying myself 90% of the time.
The answer is to change the default to Photoshop, and then just change it back. You then have the default of iPhoto editing pictures with a double click, which you want most of the time, and then for those rare good shots, Photoshop is a mere right click away. Once you set the external editor, iPhoto remembers it, and even though you switch the default back, iPhoto remembers that you like Photoshop and allows it via contextual menu. Though I tested this in iPhoto 6, I imagine this also works for iPhoto 5.
[robg adds: I think this has been around since the external editor option was added, but the behavior is definitely not obvious. I searched help, and it doesn't talk about using the contextual menu, nor the process of enabling an external editor without using an external editor. Hence, here's the tip. Probably too obvious for many, but hopefully it helps a few of you...]

