Actually, John Bishop at Cornell has done research on the mid-late 1970's New Jobs Tax Credit, which seems to be similar to Obama proposal. He estimates that the effective stimulus from that (two year) tax credit program was very large (8.7x), and their is no indication that the administration of the plan was especially cumbersome. Bishop recommends several modifications to the NJTC to avoid unintended consequences, but explains pretty clearly how the baseline for employment would be established and monitored. You can find the paper here.