old benoni defence

But would they really do that? They don't seem like forced moves to me. For example, move 2 white could simply play d5 and then either c4 or e4 after that. If they do accept it, they probably wouldn't play dxe7 on move 4, but instead play a developing move such as Nc3. But if it did work, it would be a really good trap :)

Actually, the more I think about it... 1.d4 c5 2.e4 cd4 3.Nf3!
I don't see any way for Black to take advantage of White's move order. So there are actually two ways to play for an advantage.
Unless I'm wrong.
3... a6 is somewhat irritating - against the O'Kelly Sicilian with 2... a6 people don't normaly play 3. d4.
History of benoni defence
"Benoni" is a Hebrew term meaning "son of sorrow," the name of an 1825 Manuscript about this opening. "Whenever I felt in a sorrowful mood and wanted to take refuge from melancholy, I sat over a chessboard, for one or two hours according to circumstances. Thus this book came into being, and its name, Ben-Oni, 'Son of Sadness,' should indicate its origin." - Aaron Reinganum