Navigation
As far as I can tell it doesn't cache routes. It may but I'm not driving far enough out to be out of cell range. If you turn on Airplane mode Maps stops navigating and tells you to turn Airplane mode off to continue navigation.
Whoever was asking about battery life. Hard to tell as I've been using the heck out of it. Does seem to charge slowly while navigating. I'm guessing navigation is even more of a battery hog.
Music
Also one super slick thing about the new Music app. I'm driving down the road rocking out with my son and notice that the volume and seek radio chrome sliders are pulsing. If you tilt the phone (stand it up then slowly lay it down to see effect) the sliders turn back and forth. Nice touch almost no one would notice.
Hard to tell in a screenshot but they two chrome sliders are slight different:
Safari
Also just realized that mobile Safari on the iPhone finally puts a new tab next to the one your using and not at the end.
Also, thanks to iDB.com for this one, you'll now have a history for back and forward on the iPhone and iPod touch by holding the respective buttons down( back/forward). This is already on the iPad in iOS 5.1
Wallpaper behind the web pages (when you pull on them) is linen and no longer grey on iPhone/iPod touch
The more I play with iOS 6 the more I think it's one of the biggest updates yet.