A point that emerged from the discussion of http://bugs.darcs.net/issue1096 :
patch bundles should have some sort of format information, the way repositories do.

Am I right in thinking that because we currently only have one .dpatch format,
to resolve this issue all that is needed is to hard-code Darcs to include a
Format: 1 or Format: traditional somewhere in the .dpatch?

The usual concerns about backward-compatibility apply, so this probably involves
putting it in a place that old darcs would treat as a patch bundle comment.

I can't think of a real use for this.  I need to see a plausible scenario where
an older darcs would want to apply a bundle generated by a newer darcs that
wouldn't already be covered by the repo format.

Reopening. I think this makes a lot of sense. The details have to be 
figure out, though.