On Windows, pushing to a local repository fails if the command is run from
inside a subdirectory:

[...c/darcstest]$ mkdir a
[...c/darcstest]$ cd a
[...darcstest/a]$ mkdir newfolder
[...darcstest/a]$ darcs init
[...darcstest/a]$ darcs add newfolder/
[...darcstest/a]$ darcs record -a -A me -m new
Finished recording patch 'new'
[...darcstest/a]$ cd ..
[...c/darcstest]$ darcs get a
Directory 'c:/darcstest/a' already exists, creating repository as 'c:/darcstest/a_0'
Copying patch 1 of 1... done!
Finished getting.
[...c/darcstest]$ cd a_0/newf
a_0/newf: No such file or directory.
[...c/darcstest]$ cd a_0/newfolder/
[...a_0/newfolder]$ darcs push
Local to local copy not supported
Invalid repository:  ../c:/darcstest/a

darcs failed:  (scp) failed to fetch: ../c:/darcstest/a/_darcs/inventory

(I did this using a cygwin shell, but the same thing happens in cmd.exe)
By trying the daily builds linked to from the wiki, I've found that this bug is
present in 2005-08-01, but not in 2005-07-29.

What --exact-version of darcs are you running?  This looks like a duplicate of
issue 47 which was resolved by patch "fix handling of absolute paths containing
drive letters".  Daily builds from 2005-12-28 on should contain the fix.

You're right, I was running an older version of darcs.  Sorry for wasting your time.