The biggest problem was the GUI rotation is not working. ... Am I right in assuming that this feature is not implemented yet and just has an options menu entry prematurely. Or is there a problem?
Further testing, I think modrana has a serious problem with rerouting. When routing is running and there's the blue box saying 'tap me to reroute', I'd tap it, it says rerouting, then it seems a new route is retrieved from google - not sure about it, as it was the same route I was following - but then the interface in stuck, i.e. the car position is updated on the map, but the display doesn't react to touches... The N900 doesn't freeze though, as I could close modrana via the X button in the corner.
Also, maybe there is some problem with routing: i.e. I routed to some point, I was already driving there, as I know where it's at. I was on this long, many km stretch without turns. After I got on it, hit 'start navigation' from the closest point which was some 200 m already past, as I was on the road, and now as I kept driving, the blue box kept showing me next turn, which was the first turn, i.e. to get on the longest stretch of the road, and the distance to it was growing, i.e. ran up into several km. So maybe it should not be closest point to start nav from, but closest next point, as then as in this case it can lead to strange results.
Finally, I think if you route to somewhere, navigation should be started by default. i.e. you shouldn't have to press the option to start navigation.
Hmmm, in fact I'm fairly certain the problem lay in navigation, not rerouting. Tested further, and now didn't even get to the blue box, saying recalculate. When I enter the data I want to route to, I see the route, now hitting 'start navigation', gets me into the same "freeze", i.e. the car position is updated, until it runs of the map, as it's not autocentered after touching the screen. But then modrana doesn't react to keypresses. It's not really freezing the system, as the CPU load doesn't rise, modrana can be closed, but it's dead to input. Left it like that for a couple of minutes, in hopes it'd pick up, but didn't. Before, the blue box also didn'h display right away, but in the end it did, but not now. Same problem a couple of times, so it's a recurring behavior.