In the season 4 finale Alexis wears this gown to Fallon's wedding ceremony at the Carrington mansion. Before the ceremony can begin (which it never did), Alexis is arrested and taken to jail for the murder of Mark Jennings. The episode ends with Alexis, still in her gown, hauled off to the local jailhouse, and thrown in a holding cell with prostitutes and drunkards, as she screams dramatically, "let me out of here!" This makes for a very memorable and over the top season finale! In the season 5 premiere Alexis makes her bail and is taken back to her penthouse by Dex and Steven.
If anyone has any information on the location of the actual gown please let me know!