Don't get your hopes up. UT3 for PC was for sale in store bargain basement bins surprisingly quickly after its release.. Epic are very slow to update it and rid it of bugs. And Epic staff are generally absent minded about the whole thing. And thats just for the PC version. The Mac version was slated for end of Spring 2008. Well, thats been and gone, and still no sign. Given the downward slope the company is heading in, i'd say the outlook isn't good if you really wanted to run this properly in OS X.
Whats more.. Even if they do manage to pull it off.. Im worried it will be a really shoddy port job thats going to need a silly powerful Mac just to overcome the games inefficiencies.
If you must play it, then you really may as well just get the PC version anyway, and run it in Windows via bootcamp. Because few if any PPC Macs would be up to it, even if a Mac version was available.
I don't know about Gears of War however. But given that it uses the same engine as UT3, I expect the story is much the same.