Lacking cryptographic hashes used by most DVCS software, SVN repositories need some external mechanism to be pretty sure the source repository hasn't been compromised. For instance, one might have snapshots of the SVN tree taken every week.
It's much easier with git or hg, even in the single-master-repo model, even if all developers are using the CVS gateway so they don't have local copies of commit history. All such a project would need in order to protect against compromise (ignoring physical redundancy) is a backup git or hg repo, kept offline except for periodic pulls. As long as there are no warnings about upstream rebases, and you trust all visible commits, then everything's fine[1].
With SVN, as long as there's no built-in cryptographically secure way to connect one snapshot to the next when backing up SVN repos, determining the existence of a backdoor requires comparing entire backup SVN repos against the live main repo. Even if sufficient backups exist, the process is slow and requires taking the main SVN repo offline if there's any chance of a root compromise.
[1] Caveat 1: in any RCS, there could be backdoors masquerading as legitimate-looking commits if the attacker had commit access. Caveat 2: The security is subject to the cryptographic security of SHA-1.
In this FreeBSD case, if they're certain the svn repo wasn't compromised, all they have to do is validate the checkout on the two compromised machines against the svn repo.
Nothing, if you don't mind using a centralized development model.
The BSD's tend to be ultra-conservative in regard to version control systems - for example OpenBSD is still on CVS and developed their own version: http://www.opencvs.org
I also find the ability to branch and stash easily two super-useful features. Also, I like not having to go to the server to do diffs, merge, or pull files from other branches. bisect is nice, as well.
I use git-svn at work, so I have everything but easy (svn) branching and I find it super useful.
