There's a Things to OmniFocus script at
http://forums.omnigroup.com/showthread.php?t=14846 and a thread on how to install and run scripts at
http://forums.omnigroup.com/showthread.php?t=7453.
I haven't investigated RTM or Evernote in any depth, but got the impression that both required rolling your own GTD system, and so expecting a canned importer to be able to do something useful is perhaps asking a lot...