System Information
Windows 7 64 bit
Radeon HD 6970

Blender Version
Broken: Blender 2.4+

Short description of error
If you have a model and somehow end up with a negative scale on one or more axis (mirroing etc.) and then apply that scaling the normals get inverted. This is a bit of a nuisance when working with for example game models that will be baked or rendered in other applications where you first then notice that the normals are inverted. A big part of the problem is that it looks fine in Blender with backface culling turned on until you view it in an external application.

This has been a problem with Blender for as long as I can remember so I don't know if you'll consider it a bug or not, but I do as it unexpected and detrimental to working in pipelines with other applications. The solution I'd go for to solve this would be to automatically flip the normals if a mesh with negative scaling is detected when the scaling is applied (upon export or otherwise).

The same problem also occurs in edit mode when scaling models negatively, but that is not as bad as backface culling will clearly show that there's something wrong. It would be nice if this could be fixed as well by flipping the normals upon negative scaling in edit mode as well.

Exact steps for others to reproduce the error
Apply the scale on Suzanne in the attached .blend file or export it as an .obj and re-import that while having backface culling turned on and the problem should be apparent.