The N900 maintains two different volume levels (for plugged and unplugged headphones), so to avoid public transport situations, you just have to unplug your headphones, set the volume to zero, plug them in again, and set your preferred volume. Whenever the headphones get unplugged, the volume will be muted, and when they are reconnected, the volume will be restored.

That said, there might be a use case for having headphoned the N900 as well, just have not got around to do it, as the volume feature described above fixes public transport situations for me already