I am currently maintaining darcs repositories both at code.haskell.org and 
hub.darcs.net. It would help me to have an option for 'darcs push' that 
pushes to multiple repositories in one go.

A command like

$ darcs push --all-repos

should push to all repos in _darcs/prefs/repos. I think it should be 
rather equivalent to

$ for repo in `cat _darcs/prefs/repos`; do darcs push $repo; done

Currently I am doing the same with a simple shell script. Thus the new 
option would be a nice addition, but no essential feature.

The feature as described by lemming would mean that patch selection is
done separately for each repo. I wonder if it wouldn't be more useful to
do this step (and creation of the patch bundle to be transferred) just
once. The obvious problem with that is that the set of patches offered
depends on the target repo (we offer only those that are not yet applied
in the target). For a common patch selection we could take the
intersection of all target repos. The bundle we create based on that may
contain patches that are already applied in some of the target repos but
I think that would not be a problem. Alternatively we could use an
option to use the union instead, so we'd offer only patches that are in
none of the target repos.