--- Operating System, Graphics card ---
imac OS X (10.8.4)
4 Go ram
ATI Radeon HD 2600

--- Blender version with error, and version that worked ---
Error happened on 2.68.0 official release, 2.68.1 (r58566) and earlier


--- Short description of error ---
Influence texture still influence a hair particle system after you unlink it from the Particle > Textures Panel

--- Steps for others to reproduce the error (preferably based on attached .blend file) ---
_Open start-up file
_Enable Cycles render engine
_Add a basic hair particle system on the default cube
_Go to the textures panel available in the particles properties and add a texture in the first slot
_Use the "show texture in texture tab"
_Default "clouds" texture is fine, just check the "Density" influence
_Back in the particles properties remove the influence texture by using the unlink button
_There's no more hair visible, density still influenced by an empty texture
(You can have your hair back only if you go in texture tab and uncheck Density for the ParticleSystem)