<update check="background"/> has no effect in:

http://javaweb.sfbay.sun.com/~ngthomas/javafx/updatecheck/test.html

It looks like it's due to that the line is in the end of the jnlp file. If moved to
 in between information and resources instead:

   ...
 </information>
<update check="background"/>
   <resources>
   ...

It works.

But update tag should also work at the end of JNLP file too. Looks like a bug.