Changing CCK field value(s) by clicking URL - possible? Advisable? I'm trying to figure out if it's possible/advisable to create URLs that get sent out in user notification emails which, when clicked, carry out actions like changing a CCK field value.

So for example - say you have an issue tracking system. There's no activity on one issue for a week, so the system sends an email to the person who posted the issue asking if they consider the issue closed.  And if so, please click here to close the issue. 

They click on the "close the issue" link in their email, the CCK status field for the issue is changed to "closed," and they're taken to some arbitrary page on the site.

Is something like that doable?  

In Drupal 6?
It's feasible but not advisable. Drupal's standard for changing data via URLs:
1) Use _GET variables (or simple URL paths) to _access_data
2) Use _POST variables to change data

Along with using _POST, you would want to use Drupal's Form API so that tokens and fields are generated and validated.

Options you could do to change the data via URL (e.g. link) is to:
1) Add a token to URL to validate it's only used once (or other validation necessary)
2) Ensure user is logged in (and maybe ensure he is owner of that ticket, etc.)
Hope this helps.