Cerulean Studios have announced an alpha version of Trillian for the Mac, which they have gone out of their way to clarify is not a port but an entirely distinct dev project. I’m not sure how relevant Trillian will be in a Mac universe that already includes Adium. It will be really hard for Cerulean to improve on Adium – and certainly Trillian for Windows is nowhere near the quality of Adium.