Bitbucket has a help doc on code review: http://confluence.atlassian.com/display/BITBUCKET/Setting+Up+the+Bitbucket+Rietveld+Service
A few issues with this: 1. User comments seem to indicate integration with bitbucket is cumbersome (haven't tried it myself) 2. Rietveld doesn't seem to be actively maintained and more importantly Gerrit which has more recent activity shares its origins with rietveld. From the little research I've done online looks like improved version of rietveld.
So just making a suggestion here that it'd be great if bitbucket would integrate gerrit as a service: http://code.google.com/p/gerrit/
I had signed up for an asembla account but later switched to bitbucket due to unlimited repositories for the free account. However assembla has gerrit integration and looks pretty cool: http://blog.assembla.com/assemblablog/tabid/12618/bid/40871/Introducing-Gerrit-a-scalable-code-review-system.aspx
Would be great if bitbucket could add this as well, believe it would be a better addon service than current code review feature.