+1 support for this. Space dust for a velocity vector is simply not good enough.
Especially for low-level planetary flight and FA off manuvers, this is very useful. At least give us the option to enable/disable it.
It
is an easy thing to implement considering the velocity vector is already calculated for normal flight, we just want a HUD indicator for it.
Edit: For clarification, something like this is what I would prefer (the middle arrow pointing to the circle with 3 lines sticking out):
An opposte (retrograde) indicator would be nice too for reverse flying. Think Kerbal Space Program.