SIP is a standard defined in RFC RFC3261. SIP used in any SIP compliant devices and proxies, asterisk, cisco, many others..... .
Skype uses its own private protocol and nothing is published about the protocol, you can't develop your own skype device.
Skype session messages are not clear text such as "SIP INVITE, BYE, ...." as in SIP or even H323.
So to answer your question : skype is not SIP, but you can analyze raw skype network data with wireshark, have fun ....
You may find some tips about how skype works in google search :
http://www.secdev.org/conf/skype_BHEU06.pdfDenis DIDIER
SR20 service / France
