AFAIK it requires a firmware hack rather than just a magic code sequence, which means it will invalidate your warrranty. You're best off buying it with multiregion already installed as then the shop will give you their own warranty. There may be disks on ebay etc. that do the hack - if you trust them!
yes that right you have be careful but you better buy multi-region from any online store like I bought from www.totaldigital.biz that I got 540HX multi-region -- the version showed on my 540HX is Region: 14