It is my personal opinion that Linux should be a viable option for users and sytem administrators that don't want to migrate to Vista for what ever reason they might have. Linux has become very user friendly lately (particularly Ubuntu), and it can be configured to be very Windows like to help the transition of even die-hard Windows users.
It appears that Microsoft are shooting themselves in the foot with Vista, yet many users will eventually upgrade to it because they are not very aware of the alternatives.
The main problem with Linux being open source is that there is no central money backed marketing of the OS as an alternative. The open source community is relying on word of mouth to spread Linux and the only active marketing is for support contracts of some of the distros such as Ubuntu, Red Hat and Open SUSE.

- some food for thought for Canonical, Red Hat and Novell.