Mac OS X 10.4 Tiger
is shipping, with delivery by April 29. US$129 to upgrade, but there's
a bundle with the new iWork and iLife (including the superb GarageBand
2) for $249, and students/faculty can nab it for US$69.

My OS philosophy remains: if you're an audio person, don't ever
install the OS upgrade the day it ships. Wait a few weeks and see what
bugs other people encounter, then once your drivers and such have been
updated, take the plunge. Here at CDM HQ I have two Macs and plenty of
audio interfaces, so combined with your reports we'll give things a try
and let you know how everything goes. (So yeah, okay, I'll probably
give it a go sooner and see if anything blows up. Better me than you.)

The good news is, it looks like both Ableton Live 4.11 and Apple Logic
Pro 7.1 (the latter shipping just before Tiger) will both be
ready-to-go on Tiger. See CDM's OS X Tiger / Windows Longhorn speculation
for more thoughts on these upcoming operating systems.