There is a nice thread on the Eve-O forums speculating on the landgrab that might come, once Planetary Interaction goes live: http://bit.ly/b6peyb.
According to that thread there are 194 players for each planet in hisec. But not everybody will try PI. And a lot of people will dabble in it at first and forget about it as it takes a bit of work and it might not be everybody’s cup of tea.
It will probably pay off to look for some juicy planets in low sec. I might have to dust off my blockade runner ;).