I've done some significant searching for this very thing...
I hope someone at Sencha will correct me if I'm wrong, but I don't think there is currently a way to do this. You could open up the link on a panel in an iframe, but you would lose scrolling momentum, and some sites block opening in an iframe (facebook, google) to prevent click jacking.
There is a plugin in phonegap called childbrowser that will do this for you, but I'd like to see a childbrowser like class a part of the Ext.Device group. (That and broader control over the info.plist settings.)
This feature is a must have. I hope the Sencha Touch framework can add functionality like this soon. Or tell us how it can be done now.
One of my apps pulls in syndicated content with links, and if a user clicks on a link, there is no way to get back to the app. Lack of a childbrowser also makes oAuth difficult.