Nothing has been posted to date (at least that I've found), here's a response taken from the Apple Discussion Boards regarding Tiger and hardware compatibility:
Many people are asking what the system requirements for Tiger will be. At this time, Apple has not posted any official system requirements for any component of Tiger. There was initially a list of graphics cards that would be supported for full Core Image support, but that list has been removed. What, if anything, that removal indicates, no one here knows or can say.
Apple has not posted anything further regarding supported processors, minimum or recommended RAM or hard drive space, or what other technologies might be supported. We'll all have to wait for Apple to post further information before we will know what Tiger will require and support.