4G WILL (by definition) have better coverage... once rolled out. No way we'll have 4G before 2015 though, at the earliest. The auction for the spectrum doesn't even take place until later this year or maybe even next year dependant on court cases currently ongoing.
3G doesn't have the coverage for this, and UK data usage plans are far too low to even consider this for normal navigation - it could be useful for a single glance though, or to show what to expect at the next junction, rather then all the time.
It's a shame they can't build any part of this into the current A3's though - the new one looks okay, but too much like a stretched A1 to hold my interest for 18 months or more while they bring out the new one to match the spec of mine on order. The RNSe is probably going to be the one part of my car that I feel lets it down, though. More because of the lack of 7 digit postcode search than anything.