Incidentally my sister just got a paper published in the New England Journal with a similar approach (kill switch) for use in gene therapy http://www.nejm.org/doi/full/10.1056/NEJMoa1106152

This could be pretty important work to make gene therapy much safer.

   A single dose of dimerizing drug, given
   to four patients in whom GVHD developed, 
   eliminated more than 90% of the modified
   T cells within 30 minutes after 
   administration and ended the GVHD
   without recurrence.