As of today smplayer and smtube cannot play or download youtube videos. This is not a bug exactly because both programs worked fine even as recently as yesterday. It looks as though youtube may have changed their system, perhaps to deliberately thwart smplayer. I can play youtube videos using Firefox with the flash plugin, so the videos are OK.
Same here. Probably YouTube changed their API again. Even though youtube-dl is not affected (or it was already fixed). In the debug output it looks like there's something in front of the url:
"quality=medium&itag=18&type=video/mp4%3B+codecs=%22avc1.42001E%2C+mp4a.40.2%22&url=http://r1---sn-[...]"
I think it's fixed in svn r4912.