I haven't used 2003 much, so I can't answer that question.  Here's how I do it in blackpearl.  If your activity has a client event, the wizard automatically created an activity level variable called Outcome.  The line rules will be set by the wizard to reflect one of your actions, for example, Outcome = Approve.  Edit your activity level variable and give it a default value, like Expired.  If the user makes a choice, Outcome will contain the chosen Action.  If the activity expires, it will still contain your original value.  You can then add a new line (or modify an existing one) to check for your default value.

If you're not using a client event, you can add your own activity level variable.  Just be sure some event sets it to an n appropriate action if it is completed before expiration.

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