i really wanna know this one...
is it when i log in to the network or when i become a node on the skype network...
cause to my knowledge the login and the data handling is different...

Why would a P2P program need to know your IP address, only to log you into SKYPE server data base possible

You log into the network with your IP address or that of your router.
Skype knows your publicly routable IP yet the first second when you see your on line staus i.e. avalable ( in green).
And you can see the IP of the super nodes than you are in contact from the very first second of you your logging.
Use Ethereal.com to sniff/see the IP addresses of all the computers that the yours is in contact when you run Skype.
Look at shared.xml file in your computer to find out a long list of IPs.