I know this is years later but this just helped me- thanks all this thread is good. Was having trouble exporting directly from a .ma file to ZBrush and tried .obj instead and it worked great! I think I will try that adding that script down there to my custom shelf too!
i still seem to be having issues with this as well . back in 2009/2010 ,once you have set up obj export , it was the default name in the export dialog box. Now in 2011/2012 even when i have set it up the default is ascii. and everytime i want to export a obj i have to use the scroll down pop up window to find obj. Once a day isnt a problem , but i use this command 20-30 a day and its starting to become VERY annoying..