Windows Live XMPP and iChat
A lot of blog sites seem to have assumed that Windows Live's XMPP support would work with any existing Jabber client, but it isn't so.

To log into the Windows Live Messenger network with XMPP, a client program needs to use OAuth; i.e. it has to direct you to Microsoft's web site to log in to Windows Live, and then it has to connect to MS via XMPP and pass a token it got using OAuth. 

So, to use iChat with it, either iChat would have to add support for all of that specifically, or there would need to be some kind of proxy in between iChat and the MS servers that would do the authentication part and then pass everything else through for iChat. Sadly, I haven't heard of anyone building anything like that yet.

If all you wanted to do is to connect to the Windows Live Messenger network from iChat, you might have more luck just going through a Jabber server that has the MSN Transport installed, which connects to the Windows Live Messenger network using the old fashioned MSNP protocol instead of XMPP.