XiphQT
The wait is over! XiphQT is here. It's got a little room for improvement, but it's a proper QuickTime codec for QT7 (e.g. Tiger) that plays Ogg. In particular, iTunes' ugliest corners show through. Streaming is a bit broken, and there's some metadata problems. But this is primarily iTunes' fault, and as it's closed-source there's not much one can do about it.
So now that I have more RAM (didn't I tell you?) and Ogg support, I actually sometimes listen to music on my laptop now. But mostly I listen on the desktop with my nifty XMMS xml-rpc remote I hacked up in no time flat thanks to ruby. As a bonus I get IMMS there too, although it's contaminated with my wife's preferences too.
I hear tell that VLC supports Ogg and isn't half bad. I'll have to give it a try tomorrow. If I like it you'll hear about it. If I hate it you'll hear about it. Otherwise you'll know I'm still listening to Sugar Tax on the desktop speakers...