Ok, not quite any mobile, but as long as you have a phone with a web browser, you can have the power of Gizmo5, formerly Gizmo Project, available to you.

Visit giz5.com from your mobile phone and either sign up for a new account or sign in with your existing gizmo5 account. You will have instant access to all your IM buddies from MSN, Yahoo, AIM, Google Talk, and others.

The one thing it doesn't do is automatically refresh the page in a chat session. Certainly possible to do that with the Nokia handsets, but don't know how well it works (or doesn't) for others.