I was just browsing through the Marketplace and found this app called Nav Launcher. It lets you start Google Nav with a destination address pulled from your contacts or from a custom-built favorites list.
I've barely tested it, but it definitely seems like the sort of thing people have been looking for to make Google Nav better, and I haven't seen anyone mentioning it here, not even in a search. So hopefully it works as well as it looks like it should.
It also looks like it keeps track of your Nav history for everything you start in the launcher.