My sister's wedding was in November and there was a couple of amazing things that happened with respect to wedding cake.
First off, I'd say that yes, there is a difference. One of grade. Regular cake I would liken to what you get at your neighborhood steakhouse when you order your favorite cut, USDA choice meat. In comparison Wedding cake should be the equivilent of Kobe beef.
Now the odd things that happened at my sis's wedding. First off, the grooms cake was done by a professional baker. It had custom designs for the icing, was expensive, but was basically a sheet cake that cost as much as the brides cake. Now the odd part. The cake was TERRIBLE. With a hundred something guests that cake didn't get more than a quarter served. I doubt if more than half of what was served was actually eaten.
In comparison the bride's cake was beautiful. Gum-paste flowers (more for looking at than eating), custom icing (color and a bit of design), Tasted absolutely amazing, fairly certian everyone there had at least a slice, and many had two or (burp) three. (I wound up having three or four slices of what was left over over the next couple of days...) And that goes to show the size of the thing.
The odd part about that? Two words: Wal-Mart. Yes, Wal-Mart makes wedding cakes, and dang good ones too.