I'm sure you're correct that it could be a copyright
infringement claim, but it could also be a patent
infringement claim (
http://en.wikipedia.org/wiki/Software_patent ).
Moreover, unless you're an intellectual property lawyer, I
wouldn't be so sure about your conclusion. However, if
you're absolutely risk averse - then I can understand why
you would see it this way.
BTW, don't forget that MS released the CLI back in 2001-
2002 under a Shared Source license - and this did indeed
run on FreeBSD. Also, it would be worthwhile to read
some articles on boycottnovell.com and groklaw.com to
get a better sense of the legal risks in using Mono:
http://boycottnovell.com/2007/11/29/mono-microsoft-license-patent/
http://www.groklaw.net/articlebasic.php?story=20070614231022599
