I just wonder why my Panasonic PV-GS400 (NTSC) can not playback PAL recorded tapes? OK, not fully correct - actually it plays the tapes, but shows a red message on the LCD that it's unknown format or something like that. My friend's NTSC Sony cam (sorry, forgot the model name) can at least play back my older PAL miniDV tapes from Europe. Is it really such a big issue to integrate a PAL playback function in Panasonic cams?
How is it vice versa - does anyone have tested NTSC playback on a PAL cam?

Some of the Sony PAL models could play back NTSC DV, but not convert it. My MX300 PAL cam can not play back NTSC. So this playback is a Sony thing, just as frame mode is a Pana thing.

But technically it should be not such a big problem to integrate a PAL playback function in a miniDV camcorder, as the tape speed and data rate is the same as NTSC, right!? Technically it's just a file to send to a firewire port. I don't need it displayed on the LCD wether any conversion is needed by me, I just want to copy some PAL tapes to my harddisk and edit them. The final outcome should be a PAL DVD then. Isn't there any hack or trick for the Panasonics to enable outputting PAL and NTSC?

There are no "hacks" that I know of. You could always buy a very cheap/used PAL miniDV cam to solve this problem.