There's just not enough of a proved market for such a product. Sure the MAC mini may make a decent carputer, but relativly few people are still going to go through the trouble of doing that, particularly when most of the people installing carputers are WinTel experts and a MAC mini still isn't easy enough to install in a car for the average non-tech person to do. You could target laptops, but the question is why target MAC laptops that make up a very small percentage of available platforms? Even with all the navigation competition you'd still have more of a market targetting windows.
Considering it takes at least 100k to license good map data for the US, your not going to find any built from scratch software products from small developers.
The only way I realistically see anyone developing a product like this is if there are substantial and proven numbers of people who have MACs ready to use in thier cars or there is another reason besides carputers for a company to make a realtime Nav for a MAC.