All the Nexus sites have experienced issues sending out the validation emails for new members (or any emails for that matter) over this weekend. I apologise for this problem that I now believe to be fixed. I can't possible reply to all the emails sent in about this issue so sorry about that too!
If you try now it should now work. If you registered today then you will need to wait 24 hours for your unvalidated account to be deleted so you can sign up again. Alternatively try logging in to the forums and clicking the "resend validation email" link.
Thanks to all the people who emailed me letting me know of the problem.