is Android single task or multitasking and multithreading? I am trying to understand why I cannot keep listening the audio of a YouTube video on my Galaxy S 2 while moving to another application for example reading a wikipedia article on the browser.

is there any architectural limitation which prevents this in Gingerbread (hopefully will be removed in ICS) or is only the way Google made the YouTube application?

Edit: tried out YouTube application in ICS 4.0.3 today and behaves the same as in Gingerbread :(((
It's not a limitation of Android, but a "feature" of the Youtube app to conserver resources. 

If you want to use Youtube as a music player, I think you should be able to find an application that allows you to do exactly that in the market.

TubeMate lets you listen to only the audio of a Youtube file (you have to download it in advance, though), and that should work in the background.