On my MacBook Pro, how can I open a picture in Photoshop that is in my iPhoto library? I have imported a bunch of photos from my iPhone into iPhoto. I notice that it saves my full photo library as a single file (12 GB in my case) and when I open iPhoto it works fine.

I now want to open a particular picture in Photoshop (CS 5) on my Mac and I cannot figure out how to choose a specific photo (given my whole library is just a single file).

Is there some special way to do this? It seems like everyone who uses Photoshop and iPhoto would have this issue.
You can set Photoshop (or any image editor) as an external editor in iPhoto. (Preferences -> General -> Edit Photo: Choose from the Drop Down Menu.) This way, when you double click a pic to edit in iPhoto it will open automatically in Photoshop or your Image Editor, and when you save it it's sent back to iPhoto automatically. 

This is the only way that edits made in another application will be displayed in iPhoto.