AFAIK, you can choose to include the apps on either platform through iTunes. I'm constantly having to delete stuff I got for the iPhone on the iPad, and vice versa.
WRT Navigon, I don't know if there's anything actually preventing installation, but remember that to use the full display you'll have to pixel double, which may or may not be suitable.
I purchased the US packs (My Region-east/central/west) when they were on sale @ 20 each(now $30 each). They look fine on the iPad but are made for iphone.