It's possible that someone working on the iPhone 5 leaked a real prototype to an outsider that quickly made an impression of it and returned the prototype. That impression could theoretically produce unlimited dummy molds which could then be sold to any case OEM willing to pay the asking price.
I guess that I'm making the case (no pun intended!) that the OEMs must think that the prototype/molds/whatever that they've seen or purchased are real. Real enough to invest real money into being the "first on their block" to have cases ready.
Would you tool up and spend real money fabricating actual cases if you weren't pretty darned sure that the molds were from a real iPhone 5? Silicone cases either fit perfectly or not at all. There's really no in-between. I think that it's foolhardy to think that someone could fabricate an iPhone 5 case from pictures, mockups and sketches and have it actually fit the final product.
So either these case OEMs know that they'll fit the final iPhone 5 when it ships or someone sold them a bill of goods.
Tip: Ryan Kaplan