I really have no idea but I think the main difference is the 730N has voice control. Over simplification I know. The reason you don't see them at the dealers is because orders opened in November and the 730N was not available for order until sometime in January. This means that if the is here, it was ordered before the radio was available. I ordered my car with the 730N and it is still at least a couple of weeks away.
On second thought, orders might have opened in October, but that is not the point. I also think that cars should be showing up (if not already) with the newer radio soon. I think there is a good explanation or at least a link on the general topic thread in the stickys. 2011 info or the like.
Yes that is the biggest difference. The 430 has other cool features like
1. Favorite Sat songs addition
2. Satlite radeo lets you see artist and songs on stations.
3. Crappy nonetheless but Bluetooth connection for ipod
4. Much better phone connection however
5. Garmin navigation which looks blocky, but will take addresses 10 times better than 700 series. ( I had to speak in most locations because it would not take them in my old 730 RT radio.
6. No dual radio/nav view, its on or the other
There maybe more. IMO I like the 430 better because although the nav doesn't look as good, it works much better than my old 730 series