Induction motors make so-so generators. The problem is that the speed controls the frequency more so than the output voltage. Voltage is what determines which direction the current flows in AC grid tie. Sure, speeding up a motor will cause it to generate power, but not in the way desired. The power generated will buck the phase of the AC, causing increased losses. Seperate from the AC grid, an induction motor can be used as a poor mans generator. Residual magnetism will usually allow an induction motor to begin generating as soon as a load is applied while it is rotated. Efficiency is poor however when compared to PM or even field wound generators.
A proper AC generator (or inverter) for grid tie would use phase sense to synchronize with the grid power, then raise output voltage (by increasing drive to the field windings in a generator) so that generated power flow is away from the generator (or inverter) and into the grid. Any shift in shaft speed (inverter frequency) will result in the source going out of phase and power bucking against the grid power.
Bob