I've checked this on multiple installs (all on Macs). Sorting multi-disc albums with Artist by Album and Artist by Year results in tracks not following the disc number, so track 1 disc 1 is followed by track 1 disc 2, then track 1 disc 3, etc.
Hopefully iTunes 9.0.1 is coming soon!
Hopefully iTunes 9.0.1 is coming soon!