CJ - it sems to be just you and I left in this thread but I would really like the original poster to get the right information, since he/she posed the original question.
I have no problem with the maths in post #2 regarding mass flow and velocities. If you care to, just add a value of Mach 1 to all velocity variables on both sides of the equation (or any other velocity). Obviously, the equation still holds and Sir Isaac is happy. You may well end up with a negative exhaust velocity but all that means is that the exhaust gases are momentarily "chasing" the aircraft as they slow down - remember, they cannot affect the thrust measured at the nozzle after leaving the nozzle. By adding a velocity to the equation, all you are doing is changing from a static reference frame to a moving reference frame.
The key word is velocity, a vector quantity. There is nothing wrong with a negative velocity - it just signifies movement in the opposite direction, whereas a negative speed is undefined.
Consider for a moment the practical implications of your position: As soon as airspeed exceeds exhaust velocity, thrust goes negative (according to post #2) So in a steep dive, all our turbine driver needs to do is pull off the power to get thrust reversal! But his panel would have placards warning him of the extreme danger of throttling back in level flight, in case the dreaded negative thrust kicks in.. etc.etc.
I think awblains post says it all - it warrants a careful re-read. He/she clearly has a better grasp of the theoretical mechanics than I.
"If drag is very small, then a little bit of thrust is enough to reach a speed that can be much higher than v_exhaust: in fact the exhaust velocity hasn't got much to do with it, other than for efficiency."
To really prove this thing once and for all, does anyone out there know what the maximum exhaust velocity is of the exhaust gases in the engine powering the SR71 at M3? I would venture to say below M3 but of course we need the figures in m/sec, not Mach numbers.