Note that I missed from the manual, it appears this card supports normal PCI connections, just in 64-bit slots. It was noted somewhere on here most Power Macintoshes from the blue-and-white G3 forward have 64-bit PCI slots, so that's more encouraging than if it were actually limited to PCI-X slots specifically. However, the manual doesn't mention Mac OS or OS X compatibility or show any procedures for using it in a Mac at all, so unless someone actually has one, you'd basically need to get one and be the first to try. A quick google didn't turn up anything for me.
Also interestingly, the two pages of the datasheet I linked earlier disagree on OS support. The list of specs shows Mac OS X, but the written specs only mention Windows, BSD, and Linux. The datasheet really disagrees with itself (or lists irrelevant points) on a lot of issues.