The iPhone 2.0 software is not *officially* available yet through iTunes, but it is
available (direct link). iTunes v7.7 is also
available and required to access the App Store without using the iPhone itself.
First, note that the iPhone 2.0
completely erases everything on the phone - so definitely back the thing up first through iTunes. The upgrade was pretty straightforward and took about 15 min. Once I had it installed, the most immediate changes were 1) no more Installer.app (as expected) and addition of the App Store icon.
Go into App Store and it opens to Featured Apps; as I type this, it includes AOL's AIM client, the Apple Remote, and a number of other apps. Many of the apps are free, while others range from $0.99 to, well, significantly higher - $9.99 - $19.99 being fairly common. Games generally run $4.99 - $9.99. Some of the free apps are quite interesting: Facebook, Google Mobile, eBay Mobile, MySpace Mobile, Newsgator's NetNewsWire, and even Salesforce Mobile.
So I'm already adding apps to my phone - my initial apps include Erica Sadun's To Do; AIM; Facebook; Twitterrific (Twitter client, natch); and Erica Sadun's Light (flashlight-type app). I was able to restrain myself from adding the Phonesaber - so far. :)
More reports as required - looking forward to seeing what other apps are ported for iPhone 2.0.