As usual, too much source code to post, but here's a great article I derived from my current project on validing usernames or other "slug" entities in Django against the current URL list to ensure that the chosen slug doesn't conflict and isn't masked by the application's base collection of URLs. This allows you to write applications like del.icio.us, in which your username is a first-level "command" to the application to show your bookmarks, while validating that a requested username doesn't override the "tags," "news," or other application-specific commands:

Dynamic names as first-level URL path objects in Django

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