This is going to come up increasingly. Malware makers (of programs like dialers) are already talking about suing Norton about being flagged as viruses. Likewise Gatorsoft tends to threaten anyone who mentions that their programs are bloated pieces of adware that make your computer vulnerable to all other attacks.
Spammers have also been very effective in suing people who out them for defamation.
I hope Zango gets trounced, but I wouldn't count on it.