initially dvd-rom drives were all RPC1... RPC1 drives have no region encoding in their firmware. this means that region locking after a 5th change only happens on the software level. a simple re-format or re-installation of your operating system (like windows xp) will reset all region changes and give you 5 chances to change the region setting all over again. this was fairly easy to bypass... and there were several software out there (e.g. dvd genie) that would help bypass this so that you need not reset your PC everytime you ran out region changes.
i think it was somewhere between 1999 & 2000 when the RPC2 drives came out. these were dvd roms that aside from having a software limit of 5... the firmware itself locks itself to the region specified at the 5th change. say you have a newly installed PC with a new dvd-rom drive... and you decided to set it to region 3 (philippines)... but you slipped in a region 1 disc. the software will initially let you change it to that region. effectively leaving you with 4 more changes. but with an RPC2 drive, even if you re-format your PC after that first change (hence the number of changes to software will bceome 5 again) the drive's firmware will only let you change it 4 more times... it remembers. even if you slip it into another PC, it remembers.
if you are planning to buy a "brand new" dvd-rom drive that will read all regions without any special software or hacks ... you may be out of luck. as all new drives currently in the market are RPC2 already. sorry.
one solution is to download a hacked firmware BIN and "flash" the drive's firmware rom to make the RPC2 drive into an RPC1 drive. but doing so will void your warranty (since most hacked firmware are not sourced from the manufacturers). also since "flashing" involves writing "read-only" data on a chip, a lot of things can go wrong. and if it does, it'll make your dvd-rom work less efficiently or worse, render it useless.
another solution is to "buy" (and these are not cheap) special software that'll bypass the RPC2 setting. one great example is "dvd region-free". it works even on RPC2 drives. but it must be running in the background while you play your dvds. so make sure you have a lot of CPU power before you decided to get this. try looking for software like this on the net... most will offer you a 30-day free trial.
i hope this helps.