Note: I have asked that information like this be added to Advanced Help within Views. See http://drupal.org/node/495634
This article assumes you have a basic grasp of Drupal, Views 2 and PHP
In a recent project I needed to limit nodes returned by Views by a session variable, and found it hard to figure out how.
I created a View to show all nodes of a certain content type, and then used an argument on a CCK field to only show associated nodes.
First, using the "Action to take if argument is not present: " section, I selected "Provide default argument".
Then, I changed the "Default argument type: " to "PHP Code".
Here you can add arbitrary code to the box which will be executed like a function.
This way, you can limit certain nodes by content, such as CCK variables.