Not quite true. The Xperia x10 had a locked bootloader but the community was able to bypass it and you could install custom kernels as well as roms. The only thing was that the phone took a little longer to boot because it would load the stock kernel first to get the system up and running, then load the custom kernel.
You're right, same Moto Defy, it has locked bootloader but you can bypass it by second init and run custom kernel. It's possible!
This news is made my day