Teacher Certification and the Rhee Firings

Late last week, DC School Schools Chancellor Michelle A. Rhee fired 250 teachers for a mix of poor performance and certification problems.  Details are still flowing in as to the exact numbers, but from what I can gather approximately 110 the teachers in question were dismissed on account of being uncertified.  

I'm particularly interested in hearing more details from Rhee and others as to the 110 with certification problems.  Were these teachers who were otherwise solid, or was "certification" simply a useful technicality to pink slip poor teachers?  Were these really the only 110 uncertified teachers in DC?  If not, what standard was used to pick these 110?