you cannot install a custom ROM without first unlocking. depending on the SPL it can be either SoftSPL (no overwrite protection) or HardSPL (overwrite protection) - of course the HardSPL will serve two purposes... 1) allow you to install custom roms and 2) prevent SPL overwrite in case of flashing shipped (exe) ROM. Anyway until there's an actual ROM update (exe), its pretty useless to have overwrite protection as there are no ROMs in development that actually even attempt to write radio and/or bootloader. Thus this pretty much eliminates purpose #2 - well at least for the time being...
