Oops, forgot about spaces in the dirname.
Quite likely for Windows users to hit this problem.
diff --git a/git-flow b/git-flow
index 004aa2e..a03ba1f 100755
--- a/git-flow
+++ b/git-flow
@@ -44,7 +44,7 @@
# The sed expression here replaces all backslashes by forward slashes.
# This helps our Windows users, while not bothering our Unix users.
-export GITFLOW_DIR=$(dirname $(echo "$0" | sed -e 's,\\,/,g'))
+export GITFLOW_DIR=$(dirname "$(echo "$0" | sed -e 's,\\,/,g')")
usage() {
echo "usage: git flow <subcommand>"