A court in the US has determined "that using whois privacy on domains can be considered 'material falsification'."
Domain Name News says that "the ruling doesn't make use of whois
privacy illegal. However coupling the use of privacy services with an
allegation of intentional spamming and you've got the cards stacked
against you."
In another report, Marketing Pilgrim makes similar points, and adds
that "Simply using a domain privacy service doesn't make you a
criminal, but the ruling appears to suggest that the use of such a
service-especially in the use of sending spam emails-could be
interpreted as trying to evade identification."
For more information, see:
Whois Privacy Is 'Material Falsification'
www.domainnamenews.com/legal-issues/whois-privacy-material-falsification/6474
Use a Domain Privacy Service? There's a Prison Sentence for That!
www.marketingpilgrim.com/2009/11/use-a-domain-privacy-service-theres-a-prison-sentence-for-that.html