Done.
This is what I ended up doing. Turns out, we can't really attach an 
action to the onclose event since it's impossible to differentiate 
between explicitly closing the alert and the alert timing out and 
fading on it's own. So, clicking the text dismisses the alarm, puts up 
another notification to indicate this, and opens the appropriate url 
in a new window (if applicable).

sounds good!
Should be a new window, because if i edit for example a event in this 
moment i lose this content.
Really good question, i guess if i click onto the link in the 
notification it should dismiss it as well.
No, not so easy as i think before :-)

Actually, this already works for Nag with Horde 5.2

I've actually been working on this (for Kronolith) the past few days. 
You have two observable events that pertain to this with desktop 
notifications, onclick and onclose. Currently, the 'onclose' event 
(which is fired when closing the notification without clicking the 
link/text/open button) snoozes the event. Clicking the the 
link/text/open fires the onclick event and currently dismisses the 
event's alarm. In both cases a new notification *should* be shown 
(works in everything except safari right now) stating what occurred.

I tried implementing it so that clicking the link actually opened the 
event, but couldn't get a workflow that I liked. What was happening 
was that clicking the link would open the event (should this be in the 
current window, a new window?) and then trigger yet another 
notification. The only way to avoid that would be to dismiss/snooze 
the event first - and the user might not know which he wants yet until 
he views the event. Ideally it would be great if we could have 
multiple links/buttons in the notification, but that's not available 
in the Notification API.

Anyway, I'll do similar logic for Nag, unless you have any input into 
the workflow...

Forget sometimes to complete my tasks and the notification come up 
again, direct link to complete or open this task will be great.