System Information
Archlinux
Nvidia GTX 460

Blender Version
Broken: 2.69 release (r60991) and 2.69.10 c7ac0ed

Short description of error
False "Faces needed for useful output" warning on solidify modifier when tweaking the bevel of a curve.

Exact steps for others to reproduce the error
Based on

1. Open attached file
2. Slide the bevel depth of the curve/torus object to 0 and then back to a value >0
3. Switch to the modifiers tab. The solidify modifier has a warning about no faces, but there are faces and it's working just fine other than the warning.