For what its worth, most claims seem to be that Forrest Mims has taken his words out of context.
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.
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.
sending them into space is not actually feasible unless space travel becomes as ubiquitous as dinghies. Think about how fast you could off-planet people. Even a million at a time is peanuts.
edit: I went and looked at the numbers: to maintain a static population would require blasting 75 million people into space annually.
Easy and feasible meaning "It's easily understood and sounds good." Space travel doesn't work for a variety of reasons, first and foremost being expense.
His page on the speech: http://uts.cc.utexas.edu/~varanus/Controversy.html
Wikipedia article: http://en.wikipedia.org/wiki/Mims-Pianka_controversy