Hmm. Are you actually sure that the repos are in darcs-2 format and that you have
converted only a single branch? I somewhat doubt that this is the case, for
various reasons... Can you please give cat _darcs/format? If it says "hashed",
this is actually an old-style repository, just the content is hashed. If you have
obtained this with darcs convert (and done that correctly, without converting
related branches twice), then yes, you have likely found an exponential issue in
darcs 2 format (this I think is known to exist, but should be quite hard to
create conflicts in such a way to trigger it).

Thank you.

Anything you can do to help us reproduce this would be appreciated.

Is this another case of a conflict fight, or something else?
  http://wiki.darcs.net/index.html/ConflictsFAQ
for more details

It's also worth considering if there are other non-conflict-related factors
involved.

We should also see if we can make any more progress in documenting the kind of
things that make darcs2 still hang, and ways to avoid them, or to work around
hanging situations :-/  They should be much rarer, and I hope we (well, David et
al) will continue to make progress, but in meantime, we need to make sure folks
like you are up and running.

I sadly have to report that the new darcs-2 repo format apparently fails to 
resolve the 'darcs hangs forever' bug. At least not with the two branches I 
am working with:

aragon: .../d2/pitz-work > time darcs pull -a ../mls
withSignalsHandled: Interrupted!

darcs pull -a ../mls  86398.50s user 159.97s system 99% cpu 24:02:56.79 
total

(I killed it after it ran 24 hours.)