I know there is bulletproof hosting but are there registrars who won't give out your whois info?
internetbs.net
Yup, haven't personally experienced it (and hopefully won't have to). But internetbs.net were the only ones a majority of BHWers use from my knowledge.Have you personally experienced it or is it based on hearsay.
There is a thread on BHW where one member was asked by these guys to remove their site from spamhaus database or they will cancel the domain.
I have read that prq.se is the right registrar for this kind of thing.
Ultimate anonymity would come from registering with fake details and using some VCC which can not be traced back to you.