An exploit has been discovered on the Sony Xperia Z that allows users to root the phone even if their bootloader is locked. Most phones bought directly from some carriers such as O2 and Three in the UK will have locked phones, which means they won’t get to dabble with custom ROMs. However, this exploit means that they can at least enjoy a rooted phone.
The exploit was discovered by @goroh_kun who provided a rootkit. Another user, @huhka_com then packaged an EasyRooting Toolkit for people to use. The exploit is designed for use on the Japanese DoCoMo version of the Xperia Z (SO-02E), but western users of the Xperia Z (C6603) are reporting success too. More details (in Japanese) can be found here.