I'm happy to report that it's possible with fairly minimal effort to get Markdown.pl to function as a service in OS X. This makes Markdown functionality available inside of Cocoa text fields, which means that everything from Safari to Mail to SubEthaEdit to Stickies can benefit.
First download Gust's HumaneText service.
Install it, then, using Terminal, navigate to
Copy Markdown.pl to this directory. Make it executable with this
chmod 755 Markdown.pl
This is the directory that contains the text filters used by the
HumaneText service. The service chooses which filter to invoke by
means of a simple symlink. Delete the existing symlink:
and recreate it, pointing to Markdown.pl
ln -s Markdown.pl filter
Tested under OS X 10.3.3.
People who push both buttons should get their wish.