Hi!
Seems like using labels like Today and Yesterday isn't such a good idea. I wouldn't be surprised if there's a gconf option to turn it off. If there is, there's still the question for the best default, of course.
Would it be useful to do like many email clients do: if the last modification was made before today, show "Yesterday" or the date, and if it was made today, just show the time? Cheers, Manu