If you've tried everything (.htaccess, php trick) and your web server doesn't support jnlp (like earthlink) you can always do one of two things:

1) Have them run webstart from the command line and point to your URL:
javaws http://home.earthlink.net/startit.jnlp

2) put the above command in a bat file they download and run.

It's not pretty and not the best, but at least you can webstart your programs even if the web server doesn't support it directly.