Mmmm . . . it does when I access the mobile site on an iPhone using iCab browser (set to be recognised as Mobile Safari).
(I have never used the App so it's not that.)
I haven't accessed Mumsnet on a computer for a while so maybe I am getting muddled up.
But I don't ever remember having a problem before in identifying which post is bookmarked.
I have been scanning up and down the screen to spot the "greyed over" bookmarked post when I go back to a thread.
Am I dreaming that it ever did this on the Mac Desktop version?? 