ShackNews had an interview with Cevat Yerli from Crytek about Crysis.
Shack: How significant are differences between the DX9 and DX10 versions of the game? Are there any actual gameplay distinctions?
Cevat Yerli: For single-player the difference is only in visual quality, there are no gameplay differences. Visually the imagery has more depth though 3D post processing, looks more cinematic through motion blur systems interacting and surfaces are more crisper in detail and 3D. The lighting and post processing goes through an extended next-generation HDR rendering system.
In multiplayer when you qualify for very-high settings, that is high-end DX10, you will experience tangible gameplay improvements that actually make tactical difference and lets you feel like you play single player in terms of cinematic experience.