Just leaving this up for others who might have this issue. Turns out a simple re-set to factory settings on my Keystation Pro 88 (which I purchased used) solved the problem I posted below. Whatever the former owner had programmed in it was causing the cut off notes, even though they displayed as being correctly recorded in the MIDI or Instrument track in Pro Tools. Here was the problem:

I just started using my new GPO4 yesterday installed to my Pro Tools HD2 v8.0.4. I record a track with my M-AUDIO Keystation Pro 88 and it triggers samples correctly and I hear the performance correctly in real time, but upon playback only a brief click or staccato note is heard at the attack of each note no matter how long the note shows it was played. If I double click on the MIDI or Instrument track to get the MIDI Editor window and move any note in the time line or to a new pitch, it plays back that note perfectly. Not cut-off and for the original performance note duration. If I move the note back to it's original recorded location it again cuts the note off.