I wouldn't say you couldn't play them; I just don't know why you can't. Maybe there is a website that tells you how to do the regionless thing on that particular MATSHITA (Panasonic?) DVD-R UJ-857 drive.
Later, found this website:
http://macnewbie.wordpress.com/2006/04/21/...ows-xp-machine/Not a "very" satisfactory solution! Apparently many drives and players are region free now:
QUOTE
Not only non-Mac PCs, but even DVD-players and home theater systems for homes are totally region-free, and advertised as such. It seems that Macs are the only media devices left that have this arcane and insane locking mechanism and can’t be circumvented. One dude on the net has advertised a solution to the problem which basically says “Windows to the rescue!”: he actually uses VLC on Windows XP to output to a UDP stream and access the stream from his Mac using VLC. I can’t tell you how much that hurts. -- from a comment in the above link