Metro IE is a special app. You cannot invoke an executable from Metro style apps.
Metro UI is incapable of executing any desktop applications, except for the "special" case of Internet Explorer. That means you can't have a Chrome, Firefox, Safari, Opera or any other tile--just IE. To me this wreaks of trying to lock in the browser market. Sure you can run your favorite good browser from the desktop but for all intents and purposes with as much focus they are putting on Metro they clearly intend for Metro to be the primary interface. Primary meaning the interface regular Joe users will (only) utilize.
The more we find out about Metro UI the more I'm hating it. Hate is a strong word, but in this case I'm afraid its deserved. Under the cover of its "fresh" and "innovative" appeal there are some really seedy forces like monopolization and censorship [censorship coming from the inability to sideload apps for Metro--everything must be approved and delivered through the store].
What Id like to see to resolve this issue, is a simple implementation of having Metro app require permission from the user to launch desktop apps. The permission could be granted on a case by case, range of time or permanent basis.