If anyone had any issues accessing the site over the last 24 hours or so, I finally tracked it down to a DNS issue that was affecting users of Google DNS in certain zones. The issue was caused by a rouge DNSSEC entry left by the registrar (GoDaddy) when they did an update which caused some DNS lookups to fail.
Hopefully it should be fixed now but may take a while to ripple through to everyone due to caching.