I've had 5/8" maple laid down and racked for about a month. I started shifting the boards today in preparation of beginning to nail them down and I have a couple boards that have warped. Is this cause for concern?

Also, I'm installing this floor on the second floor, with an unheated basement below. I laid down 15# felt. Is that sufficient as a vapor barrier?

A month? Seems like too long to me.

If you leave a floor racked out that long before nailing, you're bound to get some banana boards. They may have been bowed right from the beginning anyway. Just put them aside or try to bang them in. Or cut them for starters. Culling out funky boards is part of the normal install process. I wouldn't worry about it but nail that floor down. It's been racked out plenty long enough. 15 lb. asphalt felt is NOT a moisture/vapor barrier. Test the subfloor with a wood moisture meter. Then test the flooring. If they are within 2% of each other, you're good to go. But still use the felt paper. Just don't call it a moisture or vapor barrier.

Thanks. Just out of curiosity, why does it matter how long boards are racked before being nailed down?

Wood wants to return to the MC it was prior to drying. Leaving it exposed and un-attached for prolonged periods is asking for the floor boards to gain too much ambient moisture. And lose their fine milling. Some can swell to the point that the T&G's will no longer fit together. I've seen it before.

Particularly engineered hardwoods!