I've now spent a good deal of time hunting and searching through the updated SpringBoard executable. Much of what I found is dull; some is useful. While hunting around during the adding-Apps-to-SpringBoard effort, I discovered a number of items that appeared to be new Safari-style protocol handlers.
These items were each listed near a host application, for example
com.apple.MobileStore. Not surprisingly, opening an
itms:// URL launches the iTunes mobile store. Here are the working protocols, both old and new
itms:// Launches Mobile iTMS store.
maps:// Launches Google Maps. (iPhone only)
feed://, feeds:// Launches dotMac reader
tel:// Dials a number, specified in the URL. (iPhone only)
mailto:// Launches the iPhone mail app. (iPhone only)
youtube:// Launches YouTube.
Other items that were listed but do not seem to work are: calshow://, callto://, vmshow://, prefs://, nowplaying://