We've learned many times that your source code and its history are the thing you care about. Spreading closely related information across different systems is dicey at best. It never seems to follow the same branching structure as the source code, and external systems come and go. Even for issue trackers, one of the most obviously useful tools we have for software development, it's not obviously a good idea to refer to issue ids from source code comments.
That is to say: if you believe that code reviews are an important part of the code, or at least sometime can be, then you should be getting that information back into the code itself. Show me a way to turn code review into a code comment from the review interface, that would be cool.