Usual caveat: Only the SE engineers can answer this question, but a) they're not talking; and, b) they are know to rethink their decisions, so it's a case of "here today, gone tomorrow".
Privacy may be a negative but only as a piece of a larger gaming puzzle: Privacy + certain neighborhoods/topics + rate of link growth +
Privacy, alone, would not be a rational decision since there are many valid reasons to excercise a right to privacy BUT if you excercise that right to privacy then presumably you are taking other steps - such as not showing a phone number, mailing address, etc. - that would be consistent with a need for privacy - HOWEVER, (and this is a tough call) should the act of hiding behind a veil of privacy affect the trustworthiness of a website?
IF I were a search engineer I'd use privacy as a trigger for running a few other filters to look for other bad signals.
You sending any other bad signals, big guy?
But before you do that... sign-up with a DEA (Disposable Email Address) service. For a very small yearly fee, these services let you generate a unique email address for each web site registration, each online store registration, each mailing list you subscribe to, etc. When you get spam, you know where they got your address. And you can turn off individual addresses.
And they're super-nifty for domain registrations, as well.
Some people use free email accounts for this purpose, and dispose of them after a while. A DEA service is a better implementation of the same idea.
By changing the address at intervals (I find once or twice a year is normally enough), and making sure the previous addresses just bounce, you should greatly cut down the spam.
In other words, the contact address should be a throwaway address that you change when it has been harvested and resold too much.
(jtara typed quicker than me!)
So the question you should be asking yourself is:
What is more important to my business - proving that I am the rightful owner of my domain by listing WHOIS details or hiding behind a privacy service to avoid spam and potentially putting my business at risk?
I just get to mutch spam
How much is too much? OK I know, one is one too many:-(
I have 16 e-mail accounts and they receive on average 600 mails per day with only 2% valid! I know this since I went away for 5 days and came back to 3,062 emails of which 62 were genuine and this is quite normal for me.
I use a small program accessing my mail directly on the server and delete there and then download genuine mails as normal. It only takes a few minutes in a morning and then occassionally during the day to keep them clear, it also works from a flash drive too.
I do know one company that gets 30,000+ per day, yuck!