Well actually PG isn't the best mapscript to test that with, infact last month i took it out of the mapscripts, but AIAndy said there was nothing wrong with it, but like you i had problems with it also, "I'm Just Sayin."
Hm, it is the only one that can go bigger than Gigantic (it has a size override option), and I know that Gigantic Maps work with Viewports fine. Oh well.