I've been a proponent of terraforming ever since I played Buck Rogers: Countdown to Doomsday on the C64. I've just always thought it seemed like a logical progression once we've finished jacking up Earth.
Seriously, though, I can understand the ethical issues involved here. Who's to say whether these "lifeless" planets aren't simply progressing on their own evolutionary course. Our tampering could cause irreparable damage to future ecosystems - probably in the form of McDonald's.