On Android 1.0 there was a com.google.googlenav namespace for driving directions:
Route - Improved Google Driving Directions
But in newer SDK it was removed by some reason...
Android: DrivingDirections removed since API 1.0 - how to do it in 1.5/1.6?
On BlackBerry there is also lack of APIs for such stuff:
how to find the route between two places in Blackberry?
csie-tw gives a workaround (query gmaps for kml file and parse it):
Android - Driving Direction (Route Path)
Also Andrea made a DrivingDirections helper classes for Android.
I wrote a little helper for this functionality, in j2me, so I would like to share my samples on Android and BlackBerry.
UPDATE
As it was stated in comments, it's not officially allowed Google Maps APIs Terms of Service :
Google Maps/Google Earth APIs Terms of Service
Last updated: May 27, 2009
...
10. License Restrictions. Except as expressly permitted under the Terms, or unless you have received prior written authorization from Google (or, as applicable, from the provider of particular Content), Google's licenses above are subject to your adherence to all of the restrictions below. Except as explicitly permitted in Section 7 or the Maps APIs Documentatio