I didn't see a dress on a hanger, but I have seen the sweater on a hanger made by bonniebakes. Here is the link:
http://www.cakecentral.com/cake-photo-21015.html
I hope this helps. If not try googling "wedding cake on a hanger".
it's not your imagination... i too remember seeing something like this. BUT i can't remember if it was here or at another site. I'm off to search around, i'll get back to you if i find it. I remember thinking how they do that hanger thingy. 
edited to say-
ok so maybe i was wrong, so far this is the only thing i've found:
http://www.cakecentral.com/modules.php?name=coppermine&file=displayimage&meta=search&search=dress&cat=0&pos=317&search=dress
This isn't a wedding dress but hope it helps.
http://cakecentral.com/modules.php?name=coppermine&file=displayimage&meta=allby&uname=dky&cat=0&pos=22
Not sure that will come out to a clickable link or not, I don't know how to make it one if it doesn't. Guess if it doesn't you can copy and paste it.
Hugz,
anothrcookiejar
One of our members, NancysFancy, has a photo of one on her website:
http://www.flickr.com/photos/nancydidion/286876935/in/set-72157594167617683/
I love her work!
but I think her husband makes her cookie cutters... I've made my own cookie cutters for adress hanging on a hanger. I bought one of those cutter makign kits, but if you don't want to do that, you can cut them out using a pattern. Cake_Princess worte a great tutorial on how to do that (in the articles section). Also, some places that sell cookie cutters will make them to your pattern.
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