Updated 25-April-2012 at 6:25PM: People have become concerned about the 1MB limit and how it will apply to binary files in your changesets. Don't worry! Binary files don't count to the 1MB limit. Just code changes.
Review Board 1.6.6 is out! The biggest change is that you can now fetch the API tokens for your private GitHub repositories directly on the repository configuration page. This is necessary as they've removed the API tokens listing on the GitHub account pages, in preparation for a move to the new API.
Another important change to note is that we've limited the size of uploaded diffs to 1MB. We've found that diffs that are larger than this tend to bog down the server, particularly on large deployments, and that diffs of that size are generally too difficult to review.
We suspect that for the most part, people won't hit too many problems with these. The exceptions are for very large, auto-generated bits of code, which probably aren't worthy of review anyway, and very large parent diffs. If you are hit by this for legitimate reasons, please let us know. We can still tweak this limit, but want to see how this goes.
Release notes are available.