I can confirm this bug on 9.10 (64bit) and 10.04 beta1 (32bit) too.
YouTube seems to have changed their site just recently and globally broke the totem YouTube plug-in on both, my Karmic box and my Lucid virtual machine.
As this plug-in is installed by default on Lucid, this should either be fixed before final release or moved from the totem-plugins to the totem-plugins-extras package (which is not installed by default) and fixed later.
I can confirm this bug on 9.10 (64bit) and 10.04 beta1 (32bit) too.
YouTube seems to have changed their site just recently and globally broke the totem YouTube plug-in on both, my Karmic box and my Lucid virtual machine.
As this plug-in is installed by default on Lucid, this should either be fixed before final release or moved from the totem-plugins to the totem-plugins- extras package (which is not installed by default) and fixed later.