Originally Posted by Iain Lambert

. . . So, orbits fall into 5 types.

V < Vboom - you're going to crash
Vmin < V <Vperfect - elliptical orbit
Vperfect - circular orbit
Vperfect < V < Vescape - elliptical orbit
Vescape < V - parabolic flypast.

A planet's orbital velocity being absolutely bang on to have a circular orbit is the sort of thing thats really bloomin' impossible to see in a real-life situation, so everything sits in ellipses after a reasonably short while.