Review Board 1.5.1 has been released. It's a small bug fix release that fixes a few small problems, including lighttpd installation, serialization problems with the API's XML output, breakages with SCons files, and better error reporting for Git repositories with incorrect permissions, to name a few.
It's a pretty small, safe release with no real emergency fixes, but it's worth an upgrade, particularly if you're affected by one of the problems.
Full release notes are available.