Hi Simon,
One thing to remember when doing your customisation is that has to be a gap between the CWK's Period of Placement and Period of Service.
You cannot hire a new employee from their contingent worker placement and therefore you cannot run the rehire API for people who have a system person type of CWK.
You need to terminate the CWK for one day, and then rehire them as an employee.
If you wanted a CWK to be an EMP from today (16-DEC-2009) you need to terminate their placement 2 days in the past (14-DEC-2009). They would to be an EX_CWK for one day (15-DEC-2009) and then you hire them to an EMP status from the next day (16-DEC-2009).
Cheers,
Spiro