Thanks for the useful replies.
I've added eli0s ' text as a private tutorial on my computer and I am going to try out remy-jay 's tip.
Urr, I messed up the dt to seconds translation. 100 * dt = 1 sec, 200 * dt = 0.5 seconds and 50 * dt is 2 seconds. In my previews post I am way off.
Basically dt ensures that you get real time results no mater the frame rate, in our case, we want to add or subtract a value of 100 (the maximum opacity) so by multiplying +/-100 with dt we get a value change of 100 within one second. The dt is like a constant, so for half a second fade we say add (or remove) the value of 200 in dt time, but since our maximum value is 100 the result is getting a 100 value (or 0) in half a second...
Here is an example capx which basically does the fading with on mouse over/mouse out events.
http://www.eli0s.com/Tests/Fades.capx