Looks like the execs at Spirit have been talking to the folks at Europe's Ryanair.
"For customers who purchased tickets prior to December 15, 2006, for each ticketed passenger, Spirit Airlines will transport two (2) bags as checked luggage free of charge. For customers who purchased tickets between December 15, 2006 and March 6, 2007, Spirit Airlines will transport one (1) piece of luggage at no charge per fare-paying customer. For customers who purchased tickets on or after March 6, 2007 for travel on or after June 20, 2007, charges will apply to each item transported as checked luggage.
Spirit Airlines can only guarantee the transportation of one (1) item as checked luggage per fare paying customer. Additional bags can be purchased on a first-served basis; but Spirit cannot guarantee that there will be space for extra bags, and will not be held responsible for any additional items brought to the airport by the customer. Please note that space is extremely limited on certain flights."
Be sure to read their entire policy page. There you will see that they're also now charging $1 for coffee, tea, Coke, and other drinks. Even spring water. And they're no longer serving free booze in the "first class" section. The real question is will this embolden other airlines to follow suit.