
Think for a moment about the technology required to do that.  By 
the time they could make the Earth's sky look like Las Vegas, 
the people could afford to go backpacking on the Moon. Round
trip costs for 500 kg to the Moon would be about the same as
5000 kg in a Low Earth "advertising" orbit: Very roughly the
same cost as a smallish billboard, therefore. If such ads were
to become common place, that would have to be a very low price...

The night sky on a Lunar backpacking trip would still be very 
pristine... 

There's always been a problem of having to get 
away from civilization before you can really find "natural"
scenery. 100 years ago, this usually didn't take a trip
of over 5 miles. Today, most people would have to go 100 miles
or more. If we ever get to the point where we have billboards
on orbit, that essentially means that no place on Earth is still
"wild." While that may or may not be a good thing, the orbital
billboards aren't the problem: They are just a symptom of 
growing, densely-populated civilization. Banning such ads will
not save your view of the night sky, because by the time 
such ads could become widespread you will probably have trouble
finding a place without street lights, where you can _see_
the stars...

 
An ad on a moon of Jupiter would be rather pointless, since you need
a telescope to see them. However, I'd love to see them get all
the publicity they could from underwritting the "Coca Cola Io
Orbital Mapping Probe."  


They already can, to some extent: The IAU allows names derived from
sponsors or patrons of scientific research. If Microscum donates
money to a university astronomy program, one of the galactic 
astronomers could easily get a newly discovered galaxy named after
them.