Why doesn't this Views 3 contextual filter work?
In order to play nicely with other modules, views will not override any aliases that point to something already. You're trying to make a view show up on an alias that is already pointing to a standard drupal full page node. This simply will not work.

Options:


Stop giving aliases to press release nodes so they are only available at node/nid
Use a different path for your view so it doesn't conflict with your nodes
Use Panels to override your full node page and show that view


Some other things to consider
A view is really useful for display a list of things, you should probably consider not using it as a node page in this way. You can get a lot of control over your page layout using the standard display tools Drupal 7 gives you.

Using Content: Nid as a contextual filter will not recognize a url in the form of press-releases/name-of-article. You would need to use Content: Title. You also don't need to specify the raw url value as a default value or add % in your view path. The order in which the contextual filters are configured are the order in which they are pulled from the URL.