I've built an example with web-browser in windows phone 7. When I load an html file which created by myself, I can double click on web-browser to zoom in/ zoom out the content. But, when I load an web-page (ex: google.com) I can't. I uploaded my example to my skydriver account. You can't get it at http://cid-b65eb4d185de7cfc.office.live.com/self.aspx/.Public/Shared/Sandbox.WebBrowser.zip
If you known the reason why, please help me.