I know VC is not very well suited to current revision control systems, but
some of its functionality does make sense and is convenient (typically: show
diffs made to the current buffer's file, indicate the revision control
system used for the current buffer's file in the mode line, provide access
to `annotate' and .... well, that's mostly it I think).
Also, there are plans to update VC to be a bit friendlier by making VC-dired
work better.
So, in any case, Emacs-22.2 will have VC backends for Git, Bazaar, and
Mercury, and it would make sense to add Monotone to the list.
I might write it myself, but I'd first want to make sure nobody else has
done such a thing yet.