Pakistani authorities issued a circular to ISPs in Pakistan demanding that they block access to YouTube; but this doesn't explain why it's unreachable from the UK. A traceroute to www.youtube.com goes into PCCW's network and dies; the explanation appears to be that, as mentioned a few minutes ago on NANOG, Pakistan Telecoms has announced a chunk of YouTube's IP address space into the global Internet Routing Table, so traffic bound for them is being misrouted.
This may either be deliberate, or else the result of a BGP leak; whoever it was attempted to reroute traffic to YouTube from their customers to a "site blocked" page and accidentally let the new more-specific route get advertised to the world via their upstream carrier, PCCW. Unsurprisingly, we're not seeing Pakistani splash pages because, well, this suggests that all YouTube's inbound traffic just hit the web server they are using.
(So the comment on my new blog containing a YouTube link will just have to wait for moderation until PakTel either relents, sorts out the important distinction between internal and external BGP, or PCCW filters their BGP announcements.)
The politics are interesting; this comes immediately after a succession of people repressed by Musharraf's emergency rule have returned to the public eye. Talaat Hussain is back on TV, with a pair of banned journalists; their own briefly made a comeback before the cable feed was pulled again. Is Musharraf - or someone - trying to scare them, or seeking an excuse to impose more censorship?
Update: They're back...but they're back from Hong Kong.
6 pop-bb-a-ae0-0.inet.ntl.com (220.127.116.11) 36.539 ms 34.382 ms 39.406 ms
7 * 18.104.22.168 (22.214.171.124) 42.918 ms *
8 ae-24-52.car3.London1.Level3.net (126.96.36.199) 140.151 ms * ae-24-56.car3.London1.Level3.net (188.8.131.52) 454.571 ms
9 ge5-3-0-1000M.ar2.LON3.gblx.net (184.108.40.206) 34.647 ms 92.634 ms 54.208 ms
10 YOUTUBE-LLC.po1.401.ar2.SJC2.gblx.net (220.127.116.11) 164.128 ms 174.011 ms 169.919 ms
11 youtube.com.hk (18.104.22.168) 199.622 ms 162.886 ms 195.350 ms