I've now experienced this problem with two different Sony DVD players: one a very old model that I bought back in 1998 or so and one a brand new model that I just bought at Best Buy last week. The "regionless" discs that iDVD burns would not play in either player. Both Sony players returned an error indicating that the DVD region did not match the region of the player.
My gut reaction is "this is what happens when you buy hardware from an entertainment company." But Sony sells so many DVD players that I can't imagine that there wouldn't be more of an outcry if this was a widespread problem. Has anyone else experienced this? Did I just randomly happen upon the only two Sony models afflicted with the problem? Maybe a poll is in order...