How to import youtube video into premiere cs4
It's not really all that astonishing.  Youtube is a streaming media service and Premiere is a video editing suite designed to work with files on your local computer.  It isn't part of the function of Premiere to deal with streaming media or convert from a stream to a file or to stream content (though it can produce stream ready files).

To use a video from Youtube, you would first need to use a Youtube stream ripper that will save it out as an actual video file which Premiere should then have no problem opening.  When you finish your project, you would then export to a Youtube compatible format and upload the file to Youtube.  Adobe Media Encoder (which can be used for doing the rendering out of Premiere) actually has presets specifically for Youtube.