Appcasting is a way to add automatic updates to your applications using RSS 2.0 with enclosures. Mac apps TextMate, SubEthaEdit, and iStumbler already use appcasts for updates and frameworks such as Sparkle help you easily add updates for your own Cocoa app.
Application developers can describe their application and the changes present in each update. You can add CSS and images to make the update screen pretty, and Sparkle lets you specify a web page to retrieve if you would like to keep the size of your RSS file small.
Developers code the location of the appropriate RSS feed directly into the app, creating a trusted link between the application and a location on the web. Sparkle supports DSA signatures for more trusted downloads and updates.
The appcast system looks pretty cool and developers should be able to easily add automatic updates described in RSS.