What I'm reading from the NHC is that they're concerned what the system will do intensity-wise:

"KATRINA IS EXPECTED TO GRADUALLY STRENGTHEN ONCE IN THE GULF OF MEXICO AS SUGGESTED BY ALL GUIDANCE. THE GFDL AND SHIPS MODELS BRING KATRINA TO A MAJOR HURRICANE...WHICH IS NOT OUT OF THE QUESTION. IN FACT...THE ECMWF MODEL DROPS THE PRESSURE OF KATRINA IN THE GULF OF MEXICO TO 961 MB. THIS IS VERY IMPRESSIVE FOR A GLOBAL MODEL. "

Full source: http://www.nhc.noaa.gov/text/refresh/MIATCDAT2+shtml/260257.shtml

In case you don't quite understand why this is significant:
Global models are very course in their analysis of the world weather. This is becuase they are analyzing so much. The result is they are unable to predict anything eyewall or inside (and often not anything even near that). For a global to drop so much in pressure either means (a) the eye is very big or (b) the eye is a LOT lower pressure then the global model. Either way, that doesn't sound like good news.