A turbo timer is used to keep the engine running after the car has stopped, this allows the turbo to cool down whilst there is still oil being pumped round the engine. Otherwise the bearings and turbine shaft can warp and get damaged. If the turbo is oil cooled (I think most are) the stationary oil left in the turbo can 'crack' thereby causing the turbine equivalent of a blood clot.
The main problem with making a good turbo timer is ensuring that the engine continues to run but the car is secure. I think Clifford offer a turbo timer as an add-on to their fancier alarm systems.
I alway knew reading my brothers Max Power and Revs magazines would pay off some day!