(Good taste aside, Liz had actually been wearing that dress off and on since 1967. It wasn't a recent development.
I believe the first time we saw it was back in Ep #345 during the[spoiler]Burke's plane crash eps.[/spoiler]And here are some captures from the CTP/FITBs-Y2 board:
Episode #0345
Episode #0345
Episode #0345
Episode #0345
Episode #0345
)