Some drivers included in the Linux kernel used to contain non-free firmware blobs. Starting with the Squeeze release, this firmware has been moved to separate packages in the non-free section of the archive, such as firmware-linux. If such packages are installed, the firmware will be loaded automatically when required.

The release notes on this change can be found here:

Chapter*2.*What's new in Debian GNU/Linux 6.0


* thanks to forum member, fanderal, for the note about this