Out of nearly 8.5 million records in the statsum_epsus file, about 2.6 million records have a missing value for STDEV (as of October 2010 vintage.)
Of these 2.6 million records, the missing value can be attributed to NUMEST = 1 for all but 11 records. That is, the standard deviation of the estimates is not defined because there is only one estimate.
Among the remaining records with STDEV missing, WRDS found that most estimates were stopped. (See the Stopped Estimates file under Detail History.) In addition, most of these cases occur in the early years of IBES (prior to 1997) and for "International" companies (i.e., non-U.S.).