Wooo! The iTunes App Store is up! Just use Software Update to download iTunes 7.7, enable "show apps" in iTunes preferences, click on the "Applications" icon, and select "Get More Applications..." (similar interface to podcasts). Can't load the apps yet on the iPhone, since the iPhone 2.0 software hasn't been released, but you can at least browse and make a list (which I will be doing shortly).
Update: You can "purchase" applications and they'll download. This doesn't appear to use the shopping cart, but I've only tried free apps.