Does PhotoStream sync to my Mac when iPhoto is not open?
Yes! A process called PhotoStreamAgent runs at login, and idles in the background, downloading new photos when they're uploaded to your Photo Stream.



There exist both /Applications/Aperture.app/Contents/Library/LoginItems/PhotoStreamAgent.app and /Applications/iPhoto.app/Contents/Library/LoginItems/PhotoStreamAgent.app, indicating that this background-downloading behavior works no matter which application you use to manage your photo library.

(And as Asmus points out, photos are stored in ~/Library/Application Support/iLifeAssetManagement/assets)