By Phil LeBeau, CNBC Correspondent NEW YORK (CNBC) -- When Ford passed General Motors to become #1 in U.S. sales in March, I started getting e-mails from auto industry fans and players in the industry. All included a familiar rhetorical question: Is this a one-month blip or is Ford on the verge of passing GM for good to become the country's top automaker? While it's too early to say for sure, I think we're entering a period where Ford will challenge GM for the top spot month after month, unless we see an incentive war.