One good thing I have found about all this, as I use a directional antenna to beam my wireless across the road they did not correctly locate my router. Its good, because it does know I am nearby, but because my signal is beaming off in one direction it places me about 300M too far in the direction my antenna is facing, which is the middle of the local park. So unless someone knows what to look for, I could live anywhere on the outskirts of the park.
So if you do NOT want Google to know exactly where you are, a directional antenna could be a good move. They can only position you based on signal strength of your WiFi as they drive along the roads so any obstructions in that signal in any direction will skew their results.
I have to admit though, I haven't tested what it does if I stand outside right in front of the antenna, only inside behind it where the signal is weaker than it would be using the standard radial antenna. Still, its interesting.