  We analyze the weak (2 sigma) evidence for a running spectral index seen in
the three-year WMAP dataset and its implications for single field, slow roll
inflation. We assume that the running is comparable to the central value found
from the WMAP data analysis, and use the Hubble Slow Roll formalism to follow
the evolution of the slow roll parameters. For all parameter choices consistent
with a large, negative running, single field, slow roll inflation lasts less
than 30 efolds after CMB scales leave the horizon. Thus, a definitive
observation of a large negative running would imply that any inflationary phase
requires multiple fields or the breakdown of slow roll. Alternatively, if
single field, slow roll inflation is sources the primordial fluctuations, we
can expect the observed running to move much closer to zero as the CMB is
measured more accurately at small angular scales.
