The density of Venus' atmosphere at it's surface is about .065 grams per cubic centimeter.
http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html

For comparison Liquid Hydrogen has density of LH2: 0.071 g/cc.
http://www.astronautix.com/props/loxlh2.htm

So the CO2 atmosphere is slightly less dense than liquid Hydrogen.
The atmosphere on Earth at surface is:
"The density of air at sea level is about 1.2 kg/m³(1.2 g/L)."
http://en.wikipedia.org/wiki/Earth's_atmosphere

In cubic meters, Venus is 65 kg/m³, so it's about 54 times more dense than our atmosphere. Water is of course about 1000 kg/m³, so water would be about 15.39 times more dense than CO2 atmosphere of Venus.

So, in rough terms every 1600' elevation in the atmosphere of Venus would somewhat equal in terms of mass to 100' of water depth or 84,640' of the atmosphere of Earth at sea level pressure. Or 1600' of the atmosphere of Venus has more mass [about twice] than the entire earth's atmosphere [half of Earth's atmosphere is below 5.6 km [18,373']. So, in terms of mass the first mile of the atmosphere of Venus is roughly equal to a ocean of water covering the entire surface to a depth of about 300'.
Therefore, unless one dealing with high mountains, it seems likely to me that lightening on Venus won't strike the ground.