As more readers send in reports, the picture looks something like this: both Mac OS X 10.5.2 and an AirPort update for Tiger are suspect in problems causing audio dropouts and other issues on some (not all) recently-updated Macs. Serato is joining Native Instruments in saying 10.5.2 is "officially unsupported." At the same time, a number of readers on Tiger are reporting dropouts with their AirPort switched on; if you haven’t installed the AirPort update, I’d avoid it just in case, but in the meantime, try switching off your AirPort card while doing audio work if you run into trouble. 
Here’s the Serato report: 
 
Mac OS X Version 10.5.2 not officially supported 
Scratch LIVE users have reported various problems, including increased USB dropouts, after installing the 10.5.2 update of Mac OS X. 
While these are yet to be confirmed and followed up with Apple, it is currently not recommended that users update to 10.5.2. 
Users who have already updated and are experiencing problems should start a topic in the help forum to let us know the details and receive technical support.  
In case this comes as a surprise to anyone, it shouldn’t. Real-time music applications are very probably the most timing-dependent of any consumer application. If only we could get every software engineer in the world to do real-time music making. (Well, at least a fair number do.) 
Speaking of real-time, we appear to be getting this news close to real-time. Be sure to send detailed results to software you’re working with — as precise as possible — and hopefully this issue will be resolved soon. And be sure to comment here, as well, as all that data really is helpful.