I think it's possible... Skype is basically Jabber, and I believe there's an official API for it, so someone could write a plugin. There's Skype for Adium, anyways.
Skype is NOT basically jabber. It's an entirely proprietary, encrypted protocol.
With Service Plugins, I suppose it is possible for Skype (Microsoft) to create a skype plugin, but I doubt they will. Anything else that supports skype does it by reverse engineering the protocol (I believe someone recently figured out the crypto.)
There's Skype for Adium, anyways.
Skype is proprietary. I don't think it will be released as a plugin for iChat.