SilverStone EC05 SATA III with Marvell 88SE9130 Chip
Not bootable, but it is a working SATA-III card, unfortunately it is PCIe, so it can be used in the last G5 generation only.
Also, it only works under 10.5 (out of the box!). The card will show up under "serial ata" column in the sys profiler and under PCI-cards.
picture of the card and benchmarks: post #20
http://www.cubeuser.de/showthread.php?t=1456&page=2
(reading is 10-15MB/ faster than SATA-II, but writing drops down to SATA-I speed, sometimes, the user notes, but for applications and stuff, that has to be quickly read, it might be an option).
Kind of, oof-topic, but I guess, when someone will search for G5 SATA-card later, he might find this here and findit useful, if he has one of the last G5s.
Would be interesting to find out, if there is a similar card for PCI-X, though I can't imagine, that SATA-III was even produced with PCI/PCI-X.