(Correct me if I'm wrong).. When the bootloader is unlocked, it's much easier to root the device, you can change the kernel, and anything else the phone's running.
The original Motorola Droid and the Google Nexus One both had unlocked bootloaders.
And I bet those 2 phones will be the last (and only) ones to ever have an "unlocked" bootloader. Manufacturers aren't willing to do that any more for security and business reasons. Motorola Bionic will not have an "unlocked" bootloader. NEITHER will HTC ThunderBolt (their recent phones have not had "unlocked" bootloaders), BUT...the development community will probably find a permanent "root" for the device rather quickly, just like it did with the Inc, EVO, G2, etc., which allows you much more customization and control of the device.
All the Google Dev phones had unlocked bootloaders, not the Droid 1...
The ADP1 (HTC Dream), ADP2 (HTC Magic), Nexus 1, Nexus S.
And Google will continue to release dev phones with unlocked bootloaders. I doubt verizon ever will, whether it be HTC or Motorolla.