I'm having good success using 5mm subfloor ply from H.D. (a non-layout project). I thought I would have to order 3mm bending plywood (Columbia products special order from the H.D. contractor's desk) but the subfloor stuff is inexpensive and is working well so far. Same technique as GRJ's project using butt blocks to define the curve, but I use Tightbond glue to bond multiple layers of ply. End result is curved plywood. For these projects, one can never have too many clamps.
So far, I have not found voids (that will kink the bend) in the 5mm subfloor ply when I cut the 4" strips. The stuff wants to bend, and I doubt I would be happy with it to form a flat subfloor.