No, the Motorola Defy bootloader hasn’t been unlocked (yet). Rumor has it that a few individuals do have Defy’s with unlocked bootloaders though. Why is having an unlocked bootloader important you ask? Have a look at this article from AddictiveTips for a good summary of what a bootloader is, and why they are often locked.
The OP in a thread on XDA that I came across earlier today claimed to have found a method to unlock the Motorola Defy bootloader. The OP said he would be posting a video with proof of the bootloader unlock, which I don’t think ever came. I checked the thread again later and was disappointed to see that it had been removed from XDA. So, it was likely someone taking Defy owners for a ride, getting our hopes up without actually having a method to unlock.
This doesn’t mean it won’t happen. Epsylon3 is believed to be working on this, and if anyone can do this, it’s him. He recently managed to brick his original Defy, but received enough donations so that he could buy a new one. So, it sounds like he will continue working on the bootloader unlock.
There’s two threads on XDA that you should follow if you’re interested in keeping up with the status of unlocking the Motorola Defy bootloader.