This was my initial interpretation as well, but on reflection I think lessdazed meant "ask him if it's okay if his IP is checked." Although that puts us in a strange situation in that he's then able to sabotage the credibility of another member through refusal, but if we don't require his permission we are perhaps violating his privacy...

Briefly, my impulse was "but how much privacy is lost in demonstrating A is (probably - proxies, etc) not a sock puppet of B"? If there's no other information leaked, I see no reason to protect against a result of "BAD/NOTBAD" on privacy grounds. However, that is not what we are asking - we're asking if two posters come from the same IP address. So really, we need to decide whether posters cohabiting should be able to keep that cohabitation private - which seems far more weighty a question.