After updating to the new Java on OSX (JavaForMacOSX10.5)
JNLP - Files do Open with Dashboard instead of Java Web Start.

You have to:

* Right-click the jnlp file

* select “Get Info”

* under “Open With:” in the info window

* change it back to Java Web Start

You find it now in: System/Library/CoreServices/Java Web Start