Here is a CDC PowerPoint that walks through the "Isolation and detection challenges" of Non E. coli O157:H7.  Clearly, the CDC sees the fact that Non-O157:H7 Shiga toxin-producing E. coli causes human illness, but that our ability to detect it using current lab techniques and resources makes the scope of the problem less clear.  These slides do not include discussion of the multiple E. coli strains found in the 2006 Dole E. coli outbreak, the 2006 Wendy’s E. coli O21:H19 outbreak or the 2008 Oklahoma E. coli O111 outbreak.