iTunes 7.3.1 was released on July 11, 2007. According to Apple, it "addresses a minor problem with iTunes 7.3 accessing the iTunes Library." Apple also released QuickTime 7.2 at the same time.
The -50 error seems fixed
Some users of iTunes 7.3 would get a -50 error whenever their library tried to update. This is apparently fixed.
Library needs to update
As with pretty much every iTunes update, after installing iTunes 7.3.1, your Library will be updated. Also, if you load a multiple library (or a library with iTunes Library Manager) that Library will be updated as well. The update process occurs only once for any iTunes library.
Scripting changes, additions
I have not yet noticed any AppleScript changes in iTunes 7.3.1.
Several Correspondents and I have noticed that the Browser arrow for the selected track's Album or Artist field will not work if either tag starts with "The". (Admittedly, I cannot be certain if this bug cropped up with iTunes 1.7.3 or if it existed in v7.3.) Option-clicking the arrows still works to search the iTunes Store for the selected tag. I do not know if this bug is present in the Windows version of iTunes 7.3.1.
Capacity and Free Space Bugs
I only noticed this in 7.3.1, but it could have happened in 7.3: the source properties free space and capacity return 0.0 no matter what the actual capacity and free space of an iPod. I haven't checked other devices.
More as it develops...
Also see info on iTunes 7.3.