One of the most exciting things to come out of WWDC 2004 was the description of a technology in Tiger called Quartz 2D Extreme.
Introduced in Mac OS X 10.2 Jaguar, Quartz Extreme replaced Quartz to offload portions of OS X's 2D display work from the CPU to the GPU. Quartz 2D Extreme similarly would replace Quartz 2D to further offload work to the GPU.
(Click picture for video. Go to 22'52".)
However, while the Quartz 2D Extreme code is present in Tiger, rumour has that it is disabled by default in the shipping version of Tiger. Also, there is no mention of it on Apple's public webpages about Tiger. So apparently, even if the hardware includes a Shader 2.0 compliant programmable GPU (which is required for Quartz 2D Extreme), Tiger unfortunately does not use it at this time.
If these rumours are true (and we will only know for sure on the 29th), one can only hope that later point updates of 10.4 will reenable it. In the meantime, one can enable it for testing purposes using the Quartz Debug application in the Developer Tools.
(Note that Core Image is something completely different from Quartz 2D Extreme, and is included and enabled in Tiger. Core Image's GPU acceleration does share Quartz 2D Extreme's requirement for Shader 2.0 support however.)
Several people who now have the release version of Tiger 10.4 (8A428) are reporting that Quartz 2D Extreme is indeed disabled by default.