At a glance, it looks like Jimson Weed to me, something from the Datura family. As already mentioned, it's hard to know for sure until it matures, but that would be my guess. My brother has it as an ornamental "moon flower" in his front
yard, and a slightly different variety has been growing wild in my parents field for decades. I'm not an expert, but that's what it looks like to me.
http://en.wikipedia.org/wiki/Jimson_weed