Hi Graig,

Just to make sure all readers are clear about this, the NOAA and BOM criteria for an El Nino are not identical (see the BOM criteria here).  One of the differences is that BOM has a higher eastern tropical Pacific sea surface temperature threshold, so it is perfectly consistent for NOAA to declare borderline El Nino conditions while BOM does not. 

Regarding the SOI, NOAA does not have an explicit criterion like BOM does, but NOAA forecasters certainly consider the SOI and other atmospheric indicators.  As mentioned in this and previous blog posts, that was one of the main hold-ups for this event!  So to answer your question, there is no specific SOI threshold used by NOAA ENSO forecasters, but forecasters examine the SOI (among other metrics, including central Pacific OLR, Equatorial SOI in different datasets, lower and upper level winds), with a particular focus on how these metrics measure up with respect to their distribution for all previous El Nino events.  (The same is true if we have a potential La Nina event.)  

We make a final decision based on the preponderance of evidence.