There is "Maps" app available on Android Market that does exactly that already with multiple maps (including google). Only thing that is missing is step by step navigation. Not a big loss as Google Maps cannot route without data connection either.
I updated Google Maps today, and added this Labs feature (which was missing from like day 1...).
So far, it seems great downloading stuff. You get squares telling you which areas you have downloaded.
Also, since Maps can work without full GPS, it's also a good way to know your general location (which, with an SGS, it's pretty much all you'll get in a pinch... lol). I have yet to see how well this works, though. I'll probably be able to put it to the test this week, as I return home.
If this works OK, I think you should be able to tell Maps which parts of countries (or whole countries, while we're at it) is should download, besides this "spot" option, and have it sync with the rest of the data (or just when you're not on 3G). Probably one of the best simple navigation tools for Android if that happens...