So what does this mean for you as a user? As an example, with Debian 6.0 not only may your network adapter might not work "out of the box", but even your ATI Radeon graphics card with the open-source driver not work by default on Debian. ATI/AMD graphics cards require microcode living within their drivers to properly function. It's allowed in the upstream kernel and is pulled out of their proprietary Catalyst driver, but this isn't technically free.
For this and other Linux firmware components that don't comply with Debian's free standards, you will need to manually install the non-free firmware bits. At least the Nouveau developers were able to write their own firmware to replace "ctx_voodoo" on certain NVIDIA ASICs already to avoid legal uncertainties in that area.