Earlier today, we first spoke of receiving several (highly embargoed) reports detailing some of the planned features and marketing points for Mac OS X 10.4 (preliminary code-name "Merlot;" an official Big Cat codename has yet to be announced). Mac OS Rumors: Mac OS X 10.4 and the 'Year of the G5'
Not a lot of info yet -- lots of G5/64-bit optimization, Expose 2.0 "with more eye candy", and increased Windows compatibility.