How do I dynamically include ouput from an external file?
There are a couple of modules that can help with this:

Web widgets (http://drupal.org/project/web_widgets): Embed any other web site content in a widget with a simple widget API.

Iframe_filter (http://drupal.org/project/iframe_filter): you just need to add this filter to an input format and the content you create using that format will be displayed inside an iframe created dynamically.

And other iframe projects: http://drupal.org/project/insertFrame, http://drupal.org/project/iframe_page (only dev version available right now for d7).