(Resuming the shorter blog posts about features in the new vcs sync process...)
After I enabled beagle conversions in cron, I started getting some intermittent non-fastforward emails. When I poked around, I noticed that some changes had landed on
GECKO90_2011121217_RELBRANCH on mozilla-release, but not mozilla-beta. When syncing mozilla-release, the tip of this branch stayed in place; when syncing mozilla-beta, the tip of this branch was behind several commits, resulting in a non-fastforward push. We'll continue to see problems like this, if we sync branches with shared names, across mercurial repos, which contain different histories, but we're good for now.
(This is more a blog post about manual investigation+actions taken after automated emails highlighted the issue, rather than a feature of the new vcs sync process, but the emails themselves were key...)