That proposed deal caught me by surprise, at the time there was hinting around that GM was going to do something with Isuzu. The GM deal came while NAV was building the F-650 and 750 for Ford through their Blue Diamond subsidiary, and perhaps more significantly while NAV and Ford were having quite a falling out over the VT365/6.0L Powerstroke fiasco and Ford's abrupt cancellation of the 4.5L V-6 Powerstroke for the F-150. I was told NAV was at least a little motivated to screw Ford to an extent by building GM's medium trucks and closing Blue Diamond as quickly as possible. The (first) GM/NAV deal reportedly fell through because GM and NAV couldn't agree on a price for the operation, but I think NAV's financial position was rapidly deteriorating and with the worsening economy NAV got nervous. After the NAV deal fell through, I once again started hearing rumors about GM and Isuzu. Remember that GM was assembling many Isuzu trucks (gasoline powered N's and the F series) at the time. Isuzu had purchased land (Alabama?) for a U.S. plant to take over N/F series production after GM was to sell their medium/heavy truck operation to NAV, and there was supposedly some talk that the new Isuzu plant could build the GM trucks after the GM/NAV deal fell through. However, once again the worsening economy caused Isuzu to reconsider opening a U.S. plant. In late 2009 GM dropped the GMT560 trucks and (temporarily) exited the medium duty truck market. Very soon after that, an insider told me that a GM/NAV deal was not at all dead, but was put on indefinite hold until the economy improved and would be revisited.
If you talk to anyone at GM or NAV these days, the official word is the new JV medium trucks are a great success. It seems to have been a slow launch, but reports on the trucks are favorable (my employer had taken delivery of a few, I will let you know....). I am hearing rumors that more GM/NAV JV trucks may be under development.