Well, to be expected really. Try and put this into context… there have been special Windows Live Messenger servers running for the last few months to enable the new users to use some of the new features. The old MSN Messenger servers were tied into them so that the WLM and the MSN clients can still talk and communicate with each other. Now that Windows Live Messenger has been released, they all need upgrading to at least accommodate the new client.
As expected (well, duhhh) there are/were/will be teething problems. Those who can sign in, don’t sign out. Those who can’t send messages to a particular person, close the window then open again and for some reason this seems to help. If you can’t sign in, use the MSN Web Messenger which should still work.
Other things might arise – such as not being able to use certain features, and some contacts (except Mobile contacts) may not appear online, and those who are currently still talking to you may appear to be offline. Although there isn’t a time set on when this will be fixed, it should be sorted within the next few hours of this post, so by about 11pm British Summertime, and yeh, it’s a worldwide issue 
To demonstrate this easier, I’ve mocked up the following concept design:
