YouTube is currently off the air. Several people are citing a blog post describing the issue as youtube.com being the victim of DNS poisoning. However, this simply ain’t the case.
YouTube hasn’t had their DNS hijacked; Yelvington describes a wildcard match in the whois tool which let’s you see not only youtube.com but other records where youtube.com forms part of the overall domain name, but doesn’t actually affect the lookup of *.youtube.com at all, e.g. YOUTUBE.COM.IS.N0T.AS.1337.AS.WWW.GULLI.COM. Check out something similar which happens when you lookup microsoft.com.
The simple fact is that YouTube is inaccessible due to the leak of network route announcements from outside a Pakistani ISP’s network border to implement a government-sanctioned block of certain YouTube ISPs. In other words, someone messed up. YouTube can easily re-appear on the net once either the Pakistani ISP resolves their route announcement snafu, or Google shift the servers onto alternate IPs not part of the /24 block being announced. Simple.