I’m sorry Ivana but I’m so disappointed about you.
Of course there is a Ustream viewer for Windows Phone 7 and it was made by your own company Ustream Inc. The release date was October 19th, 2010. And you can even see it on WP7 marketplace.
Just try this link:
And press the link at where it says: The Ustream client can be found in Marketplace here.
Now the problem is that this application is only available for some selected markets. Now please do your work (official representative?) and ask someone at your company why it is so?
Thanks for doing your work.