Strange. I tried something similar before suggesting: I saved (using SlimDX, which uses the above function under the hood) the backbuffer texture 1920x1080 with four channel half-floats. DxTex - both the 32 and 64 bit - load them fine. I don't see such a DX10 marker with a raw/hex viewer though. Is there some trigger (say, size) which "makes" it a DX10 DDS only?
Anyway: xlope01, could you upload this file somewhere, please ?
Also: you can "test" it with the corresponding loading function D3DX11CreateTextureFromFile and render it. Maybe first try a non-DDS format.
Edit: Update: Could go up to 4096^2 and again both DxTex load them fine. @xlope: Something is still off at your side I think. A 2048^2 texture should be 32 MB in size (2048*2048*4*2 = 33'554'432). And another Edit: Wait, maybe I'm silly now, I don't know if DDS does support compression. But a 2048^2 texture saves to approximately 32 MB on my side.