I posted this over in sci.astro, but it didn't make it here.
Thought you all would like my wonderful pithy commentary :-)

What? You guys have never seen the Goodyear blimp polluting
the daytime and nightime skies?

Actually an oribital sign would only be visible near
sunset and sunrise, I believe.  So pollution at night
would be minimal.

If it pays for space travel, go for it.  Those who don't
like spatial billboards can then head for the pristine
environment of Jupiter's moons :-)
