Accept the invitation from your hotmail account and follow the prompts.
If you get lost in the mix you can access the new interface by going to:
mail.live.com
Of course this is only after you accept the invitation.
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Initial impression is good. The interface is nice, very Exchange 2003 OWA like. Speed is decent and the layout is useable.
The bad thing is that it's Hotmail, or Hotmail on steroids at least. That service hasn't been usable to me in years. You can't even create a new account on Hotmail without being SPAM'd within the first day. Hell, you don't even have to use the address anywhere and the SPAM still comes... it's like magic.

Of course, that could change with Live.