One of the problems with after market navigation systems, is that they require an external, often flip out, monitor. There have been several headunits only available in Europe that integrate a navigation unit into a single-DIN headunit. For example, the Blaupunkt Travelpilot, and the Harman/Kardon Traffic Pro are mainly available in Europe.
The JVC ND-NX5000 is a single-DIN headunit with navigation. It has a 40GB drive, where 15 GB is reserved for navigation maps, and the rest is available for audio or video content. The built-in 3.5" screen can also be used for watching DVDs or other video content on the hard drive.