Steve Roberts , Mar 30, 2010; 09:15 a.m.
In iPhoto, right click on a photo and choose "Show file" it'll reveal it in Finder.
To browse your iPhoto library in Finder, go to Pictures folder in your home folder, right click on 'iPhoto library' and choose 'Show package contents'. From there you can open 'originals' and it should be arranged by year then date. There also other stuff, like modified copies etc that you can just explore yourself.