Found with J2SE beta2 b49 on RedHat 9.0 and RHAS 3.0.

Steps to reproduce:
- On the specific OS (RH9.0 or RHAS 3.0) install JDK or JRE.
- in the java control panel make sure you have set up "prompt user" under "JNLP File/MIME Association" section in Advanced Tab.
- In the browser open location http://javaweb.sfbay/~ngthomas/association/test.jnlp
Alternatively invoke command
javaws http://javaweb.sfbay/~ngthomas/association/test.jnlp
- you should get dialog asking for mime type association
- notice that dialog is not displayed.

The specified OS has Gnome 2.2.0, 2.2.2 respectively.