It a big mess in the rich client world!
I fisrt heard about Java Web Start, then about xul and then the microsoft solution xaml
Then i started to be a little confuse, so i searched for more informations...
I found different xul syntax, implementation in many different languages, xup(w3c) ...
And it become clear that it's a big mess with this contest
As you can see there is no standard way of doing things. I'll wait until it clarify!
For me the winners in battle, will be xaml(microsoft), web start (sun) and may be xul(mozilla)
And the other will die.
[update] As someone pointed in the comment section, the guy who made the contest is making confusion between xul and xul-like xml language (there is only one xul and it is not represented in the contest page) you can see a xul example here