I was afraid someone was asking this. I hope Plextor or other manufacturers won't read this...
I found this in the MMC docs:
The use of Regionalization is limited to Discs that employ CSS. There are two places that contain region information, one in the Logical Unit and another for each media that contains CSS Scrambled Title(s). When the region in the Logical Unit and that of the CSS Title are different, the system shall prevent the playback of that title (movie).
When a REPORT KEY Command is received by a Logical Unit, in the Bus Key Established state (see Figure 23), with a Format Code of 04h (Title Key), and the region code of the current media is not playable in the current region set in the Logical Unit, the command shall be terminated with CHECK CONDITION status, sense key set to ILLEGAL REQUEST and additional sense code set to MEDIA REGION CODE IS MISMATCHED TO LOGICAL UNIT REGION. A regionalized CSS media shall be deemed not playable if the region of the Logical Unit is not set.
Most units do exactly this: On region mismatch (if the region of the drive is not set, this is a "mismatch" as well) they will not reveal the title key and abort with the error mentioned above. (Exception: Panasonic/MATSUSHITA drives will not read at all on region mismatch. Don't buy them)
If AnyDVD cannot get the keys this way, it will try to crack them with a brute force attack. This probably will not work always - you will see the message "AnyDVD has guessed the key".
Some drives, including the Plextor PX-W708, can be, well, "convinced" to reveal the title keys nevertheless. (Both my PX-W708 and my Samsung Laptop are not set to any region, I don't know, if this still works, if the drive is set to a region).
AnyDVD will take advantage of this "bug" and will not do a brute force attack with such a drive.
AFAIK no other descrambler software does take advantage of this "bug".
With Ricoh/Philips burners AnyDVD has to open & close the tray to get the key. With these drives a key archive is very useful.