we would all love that but...
Most everything done is under open source licensing. And even maemo mapper walks a fine line when using google maps. Sat Navs like the Garmin Nuvi use proprietary maps and data base. Writing the function you are looking for really shouldn't be too difficult for developers BUT, sources for maps and data bases that are open source aren't as easily done. That's why Maemo mapper doesn't use maps you can buy like Navteq, not because it is too difficult to implement, but for copyright issues.

Would I like 3D turn by turn nav without downloading tracks before leaving the house? Sure, we all would.