Is it possible for people without access to the CVS tree to obtain diffs of
   stuff recently committed?

We don't provide a mechanism for that, but someone else could do so.

   I just saw a commit in the netinet tree to "avoid swapping IP addresses".
   This might break my firewall code, and it would be nice if I could look
   at just the diff.

I'll mail you a copy of the diffs separately.