I've never encountered this error before and haven't done anything differently on this site than any other. No matter which URL I submit into Fetch as Google, I get the "unreachable page" error. My robots.txt and sitemap seem fine. Has anyone encountered this before? I can't see it being a firewall blocking the IP because I'm using the same hosting account for this site as many others without error.
I've seen some malware place redirect rules into .htaccess files or site code. The result is they attempt to block Google and issue a page redirect. You may want to put your URL into: http://redbot.org/ and see what headers are returned.