That message sounds like recycled Microsoft advertising. If you wanted to go out on a limb, you might say that like this: "Open source is not an intellectual property risk. Period. That is the conclusion of companies from Sun to IBM who have looked at it. The one exception is Microsoft who launched a PR campaign saying that it was. This is a classic FUD attack on their part, nothing more and nothing less." If not, then a more toned down version of the same would make sense. (But definitely point out that companies such as IBM who have investigated this closely concluded that there was no intellectual property threat.)

I would also suggest bringing up the worm that is circulating today and pointing out the inherent and large dangers involved in trusting Microsoft software. Point out the security risk to your intellectual property that the SirCam virus represented. Raise the infrastructure costs of relying on Microsoft software, and ask what their ulterior motive is for trying to scare people off of using free products with better security.

And finally I would suggest a direct challenge. Ask them to explain exactly how they see a challenge to their intellectual property to arise. FUD unsupported by facts, as this one is, generally doesn't survive the light of day too well. So raise the issue directly. Ask for examples of companies who have found their intellectual property rights threatened. Point out how Perl's Artistic License and Apache's BSD-based license bend over backwards to avoid creating an intellectual property issue. Explain that as long as you don't cut and paste code from GPLed products or include GPLed components, the GPL cannot "infect" your code-base.

Getting the message through may take work because of your different roles in the hierarchy. But the facts are on your side...

UPDATE
How topical. Eben Moglen, counsel for the FSF, has an article on exactly how they enforce the GPL. It is worth a read, and it may sooth some unwarranted fears...