Currently pig always does a multi-pass order by where it first determines a distribution for the keys and then orders in a second pass. This avoids the necessity of having a single reducer. However, in cases where the data is small enough to fit into a single reducer, this is inefficient. For small data sets it would be good to realize the small size of the set and do the order by in a single pass with a single reducer.
This is a candidate project for Google summer of code 2011. More information about the program can be found at http://wiki.apache.org/pig/GSoc2011