Since installing Tiger, I have moved my music collection onto an external FireWire hard drive, while keeping iTunes and my library files on the internal hard drive. I noticed frequent skipping in playback, and it was mostly in conjunction with a spike in the mdimport process (Spotlight indexing). Even though both my drives are already indexed, this brief re-indexing was causing the problem.I managed to significantly reduce the skipping by disabling indexing on my FireWire drive (named 'Fire'):
sudo mdutil -i off /Volumes/Fire
I might also have to disable indexing on my internal drive, but the skips seem infrequent enough now.
[robg adds: I have my music on a separate internal hard drive, and haven't noticed any issues at all relative to Spotlight's index updates. However, the relative slowness of FireWire compared to the internal bus may explain the difference, so I'm posting this in the hopes that someone else might confirm both the issue and the fix.]

