``I'm not going to use some unsupported hacker-ware on my system.'' This is an example of FUD #2 (outright fabrication). This is the easiest argument to counter. Point them at http://www.caldera.com and point out that they can buy a service contract there or get per-incident support there.
Due to that, the more clueful have modified their support FUD:
``Linux is supported only by small companies that don't offer the nationwide support that Fortune 500 companies need.''
``There is no single point of support for Linux.''
This is an example of FUD #1 (exaggerating a weakness) with FUD #3 (spinning a strength into a weakness). The weakness which is exaggerated for FUD effect is that Linux support companies are small. This is, in truth, a weakness, but a minor one, and one which points out the difficulties of dealing with FUD #1. The most effective counter is to point out that dealing with Microsoft Support is about as helpful as calling the Psychic Friends Network for help with your computer, and that unlike Microsoft Support, small companies have an incentive and the ability to provide personalized support.
The ``spin'' applied to a Linux strength is that having multiple support companies is a weakness. The most effective counter here is to point out that, since the Linux source code is available to everybody and not just to a single monopolistic company, they can hire any support vendor to support their Linux. That is, they are not tied to a single vendor for all of their present and future support but, rather, can choose the vendor who best services them.