As far as I know the OG Droid and ThunderBolt have unlocked bootloaders. OG Droid comes like that out of the box, the TB has programs to unlock the bootloader. Some of the other Droid phones(D2G I know does) have a program called bootstrapper or something like that, not exactly sure what it's all about though. If I had to guess it allows u to run custom ROMs. Not sure if it would allow for changing of kernels though.
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No, Bootstrap recovery does not allow changing of kernels, yet CM7 is able to run on the D2 and D2G despite that.
You can put custom roms on without the bootloader being unlocked, just not custom kernels.
As for a timeline, the X was never unlocked (at least to date, and hope is fading), this one may or may not ever get unlocked either.