Windows and Mac OS X only: Apple just made the latest version of iTunes 7.7 available for download Update: and it includes the newly-opened iTunes App Store, which is available for iPhone users who have the iPhone 2.0 Software Update. The iPhone 2.0 software has not been officially released. From the iTunes 7.7 Readme:
Use iTunes 7.7 to sync music, video, and more with iPhone 3G, and download applications from the iTunes Store exclusively designed for iPhone and iPod touch with software version 2.0 or later. Also use the new Remote application for iPhone or iPod touch to control iTunes playback from anywhere in your home — a free download from the App Store.
First, evidence of the remote playback app is in the Preferences pane, here:
Second, if you check off "Applications" under "Show" in the General tab of the Preferences pane:
And "Applications" shows up in the sidebar. Happy Mac face! Select it, though, and you initially get a "The iTunes Applications Store is not currently available" message. Sad Mac face.
The newest version of iTunes, as always, is a free download for Mac and Windows.