Meh, I think Halperin drastically overstates the extent to which the "invisible primary" is going at a slower pace this time than in years past. Yes, it's true that Pawlenty and Romney are the only two candidates who are currently engaged in "the serious travel, staff hiring, contact building and general planning that are required to make a credible run for President", but who on the GOP side was doing that in April 2006? No one aside from McCain and Romney IMHO.
Halperin also says that Pawlenty "has barely touched ground in the early battlegrounds". Which I think is a ridiculous statement. Pawlenty has made numerous trips to the early primary states, as well as many non-early primary states. He's done way more cross-country travel than I would have expected from a candidate who has a day job as governor of Minnesota.
He also lists Santorum among those who are "doing very little to make themselves known". Uh, it's not for lack of trying. There are potential candidates like Santorum and Johnson who are actually doing quite a bit to try to raise their national profile. It's just that the media doesn't pay attention to them. I think Halperin is getting the 2012 election story only partially right because he simply hasn't been paying enough attention to it.