We have already seen the Sony Xperia Z (C6603) release in a few select markets and it is expected to see a wider European launch over the next week. For those buying the handset on a carrier and looking to debrand the Xperia Z, then you will need to use Flashtool and flash the phone with the desired FTF file. The process of flashing the handset will be the same as any other Xperia device, however your current version of Flashtool (v0.9.10.1) will not work.
This is as a number of features have changed in the Xperia Z bootloader including partition changes. A new Flashtool beta (v0.9.10.1b) has been created to cater for the Xperia Z. This beta version is a heavily-slimmed down version of Flashtool, for example it cannot be used to create bundles. Expect to see a number of Xperia Z FTF firmware files pop up over the next week, a French version is already doing the rounds on xda-developers. The Flashtool beta can be downloaded from here.