Any Dell machine sold without a Windows license is considered an "N-series" machine. These machines usually come with Ubuntu or FreeDOS instead. There is no essential hardware difference, although on some machines, if Dell already knows there are no drivers for a certain piece of hardware in anything other than Windows, they will not offer that option on an N series machine. For example, I don't believe you can get an N-series laptop with a fingerprint reader or any WWAN options. In the case of the Mini 9, the only difference is the sticker.
Vostro A90, 16GB SSD, 2GB DDR2, US-INTL Keyboard, FreeBSD 7.2-STABLE
Latitude E6400N, 2.4GHz, 4GB, 320GB, Opensolaris 2009.06 updated to build 118 Dell Tech Support agent