Just to clarify (this bit me last year): many people put their email addresses in the "about:" field of their profiles, so that other HN readers can email them. Here, pg is referring specifically to the "email:" field, which isn't publicly viewable but is the canonical email address used for YC announcements.
In sum, the problem is that
a lot of applicants haven't put their email addresses in their profiles
and the solution is to
put your email address in the email field [and not just the about field].
Some people put their email only in the "email:" field, without realizing that it is invisible to others. Putting it in both About: and Email: would be nice, so that we (the other readers) can send a personal email if needed.
"... We're about to send out the responses for this cycle, and we've found that a lot of applicants haven't put their email addresses in their profiles ..."
A form of self selection in a way. I can't believe that after multiple selections this message hasn't got through yet. Only 4/16 on this thread ~ http://news.ycombinator.com/item?id=348070 (not a true indication of application) had contact emails.
Does this really make a difference? Are there any studies of gmail with addresses that are hidden vesus those with plain text email addresses for bots? The critical number is the false negative/positive amount, not the absolute number of spam. And surely spammers can already find ivan[at]tipjoy[dot]com
A good way to stop spam is using JavaScript to ROT13 encrypt the address. This way you can display it in plain text, but when a bot looks to search through the source on a page it just sees JavaScript jibberish.
TextMate can do this for you automatically.
For a site, you could just have a small back end script that looks for an email address in standard format and then encrypts it. You get something like this:
<script type="text/javascript">document.write(
"nneba\100nangbzlnqf\056pbz".replace(/[a-zA-Z]/g, function(c){return String.fromCharCode((c<="Z"?90:122)>=(c=c.charCodeAt(0)+13)?c:c-26);}));
</script>
In sum, the problem is that
a lot of applicants haven't put their email addresses in their profiles
and the solution is to
put your email address in the email field [and not just the about field].