The subject frequently comes up...
You (yes, you!) can flatten warped pans.
The technique is to heat them up, put a 2x4 long enough to go from edge to edge of the pan on to the crowned side, and beat the heck out of it -- all over it's entire length, while revolving it so the entire pan surface gets its share of whacking.
If the pan is warped so the crown is on the outside, you have to rest the lip of the pan on a flat surface, so that the handle doesn't touch the surface; a stair step is good. If the pan is warped so the crown is inside, you have to cut the 2x4 to fit -- as closely as you can but without making a big deal out of it.
Just keep reheating the pan and keep on whacking 'til you get bored. The flattening will hold longer if you do both the inside and outside, but you can get most of the goodness if you only flattened the crowned side.
You can get pans very flat this way, but the pan metal will not only develop a warp "memory," but fatigue as well from the warping and straightening. Straightened pans will warp more easily than those which were never warped. At some point, the whole process gets old and the pans will need to be replaced.
Hope this helps,