Pianka has stated that Mims took his statements out of context and that Pianka was stating what would happen from biological principles alone if present human population trends continue, and that he was not in any way advocating that it happen.
Pianka is an AAAS fellow, Senior fulbright scholar, and overall respected ecologist.
Mims is associated with the discovery institute, believes in intelligent design, and doesn't believe in global warming.
I know where I'd place my bets.
The problem is that overpopulation is indeed harming a lot of things. And the only easy feasible solution is "send them to space," only we don't have any planets we could possibly colonize.
edit: I went and looked at the numbers: to maintain a static population would require blasting 75 million people into space annually.