How can this be done? We use a technology referred to as SPNEGO (Simple and Protected GSS-API Negotiation). In the old WebSphere v5 days, you had to purchase TAM/WebSEAL because it had the add-on that could parse the SPNEGO token (similar to the more commonly known LTPA token) and automatically sign you on.
Well, WebSphere v6.1 already includes this SPNEGO parser/connector/plug-in out-of-the-box, for free! Therefore, all you need to do is configure it and off you go! Here's how it works:
So another tidbit you can use to up the value of Lotus Connections (or any other application that runs on top of WebSphere v6.1) -- free SSO with the Windows Desktop! (though I would still recommend some services to configure this...)