Rider... see if you can reproduce the problem under these circumstances.

1. Set your display dim time to 10 seconds (the blank interval is not important)
2. Sign in to Skype
3. Ensure your status is "online" (just the tick - not SkypeMe)
4. From your contacts, call a landline telephone number (ie. not a Skype-to-Skype contact)
5. Before the call is answered but while the distant phone can be heard ringing, hang up
6. Wait for the screen to dim - for me, it does not dim after 5+ minutes of waiting.

Let me know if you can reproduce this behaviour. It's possible that it is specific to non-Skype-to-Skype phone calls.