Mac OS X 10.4.9 has been released.
According to Apple engineers, it should fix a bug which caused TabView (or any other application using the Audio Toolbox API) to generate broken MIDI files on Intel machines. Since version 1.8, TabView includes a workaround for this problem, thus working correctly anyway.
I’ll test the update in the next days to verify whether the problem has been indeed solved or not. If so, the workaround will be removed from the next release of TabView (the less the dead code to maintain, the cleaner the application design!) and OS X 10.4.9 will become the minimum requirement on Intel machines, at least as far as MIDI export is concerned.