smplayer.desktop have 'Exec=smplayer %f' this makes webbrowser download a mediafile to a temporary folder before lauching smplayer. smplayer can handle streaming of URL:s so changing the line to 'Exec=smplayer %u' would avoid the temporary file and use streaming.
But would that not result in the same kind of behaviour as mplayer displays in Nautilus in bug 210350 ? I'd rather not have similar issues crop up. And no, I don't use Nautilus, so I can't test it without installing a load of Gnome crap that I don't want. If someone who uses Gnome could test the suggested change, that would help. The desktop file is actually from upstream, so I tend to a WONTFIX/UPSTREAM solution.