A guy is shopping in Walmart, sees a pile of cash on the ground, he didn't see who dropped it, so he picks it up and promptly leaves the store without making any attempts to notify anyone. Later he is identified and says 'yeah I took the money, I found it, so I get to keep it'. He refuses to return the money. Was he under any duty to report the money he found? Can he be prosecuted for theft?

There is no legal duty to report "found" money.

Whether it constitutes theft is a factual determination (yeah, I know; "duh").

I'm more interested to know who leaves a "pile of money" laying arond Walmart?!?

As the smiley face was flying around the store dropping prices, it must've dropped the wad of cash that was destined for the store's safe.

How do you deprive someone of money they already lost?

It's mine.

Since the money was found on the store's floor, does Walmart have a better right to possession?

Not to get all "common law" about it, but I think Wal-Mart's claim would be superior to the finder ONLY if the finder was a trespasser or the item was in a non-public area of the store. Otherwise, the finder's claim should be superior to everyone's (except the true owner, of course).