Why is Rudd saying this? And why now? Caveat emptor warning, speculative political tea leaf reading ahead . . .
One scenario might be that Rudd sees his way back to Prime Minister. Here is how that might work.
With Gillard and Labor suffering in the polls, in part due to the proposed carbon tax, Rudd can distinguish himself from Gillard by championing the ETS and disfavoring the carbon tax. His comments on Q&A were suggestive that Gillard never wanted an ETS in any case, perhaps suggesting that she never really intends to transition from a carbon tax to an ETS as she has proposed. The subtext here is of course Julia Gillard's promise before the election not to introduce a carbon tax and then reversal of that decision after the election.
If Labor continues to take a beating in the polls and Rudd's poll numbers continue to look more favorable than Gillard's then the "all in" strategy would be to mount a leadership challenge in the form of calling an election. Rudd would campaign on the fact that he was wrong about the ETS, come out against the carbon tax, and in one fell swoop pulling the rug out from under Gillard and the Coalition. With Labor staring down a defeat it would have little choice other than to dump Gillard in favor of Rudd.
During the Q&A discussion Rudd was given the explicit chance to dispel speculation about a future leadership challenge, and he did not. He also stuck it to Julia Gillard with a smile on his face. He did not in the discussion once express support for the "carbon tax" preferring instead to speak to the need to "price carbon." Politicians speak carefully and what they say and don't say can mean a lot.
Of course, none of this appears to make much difference for Australia's decarbonization. Palace intrigue is just that in the absence of good policy proposals.