So, the real question is, will this be a problem on your site? I would look at a site like slashdot, which is visited by huge numbers of technical users, and plenty of good reasons to have tons of logons for one person. As far as I know, they do nothing of the sort.

Comparing your site to yahoo is probably not the most accurate comparison. First, I suspect that your site is not going to be nearly as high profile. Second, consider the reason that people would want a thousand email addresses from a public email site. I can think of about twenty that arrived in my inbox this morning.

I would bet that this is not going to be an issue for you. Keep in mind that the more complicated the sign up process, the fewer real users are going to go through it.