ATI will probably release this for mac. unfortunatly, ati has a bad reputation for underclocking the mac cards, and not releasing drivers good enough to run certain features. IF they do enable the advanced features, this would be a smart buy compaied to the geforce3. It is theortically faster, and can improve the quality of images without cpu overhead.
It will be released whenever ATI feels like it. of course, they seem to want to hand the mac market over, so wait a while untill you need to upgrade desperatly with a geforce3.
if you really want it, and can wait, go ahead. ati, i believe, made a commitment to release this card for mac. hell, see if you can flash a PC one (actually, don't).