If I do something like:
$ darcs send --output=~/GhcTrees/current-ext-core.patch

I eventually get:

darcs: ~/GhcTrees/current-ext-core.patch-0: takeFile
~/GhcTrees/current-ext-core.patch-0 in /home/tjc/GhcTrees/complete-2008: openFd:
does not exist (No such file or directory)

If I substitute $HOME for "~", it works.

Actually, darcs *can* handle a tilde in filenames, and that's what's bothering
you.  Expansion of ~ to $HOME is done by the shell, as in other unix programs. 
If you put a space between the '=' and the path then your tilde will work fine.
 As an experiment you can compare the results of

mkdir '~'
ls ~
ls '~'
darcs add '~'

and ask yourself why darcs should allow directories with the name '~'.

So no, we aren't going to try to emulate shell glob expansion in darcs.

Okay, but I still think the error message could be improved. Why not an error like:

"Output directory ~/GhcTrees does not exist"

instead of the current cryptic error that mentions things like takeFile and
openFd that the user doesn't need to know about?

Setting status deferred, as this is a wishlist issue.

Best regards
Thorkil

Sorry, deferring was a mistake. Setting status need-info instead, asking for a 
discussion of whether we should work towards supporting this feature.

Thanks and best regards
Thorkil