Our own investigation suggests that while the static content will load in the browser, nothing dynamic is appearing.
This suggests a database crash – the web server is running, but nothing is being delivered from the SQL back-end.
A recent tweet from the YouTube team says they know about it and are working on the problem (probably frantically!).
More information as it comes to hand.
Update: a short while after posting, it appeared the service had returned. We will update once more information regarding the cause becomes available.
Further update: fellow iTWire writer DW speculates that there were issues with routing updates, however the true reason is yet to be communicated from Google/YouTube.