According to a new research note out today, compared to its launch-week usage share, the new iPad is now being more widely used in the United States than the original iPad, launched in April 2010.

That’s good news for Apple’s post-PC strategy, one that will hopefully put to rest concerns that the Retina display, faster GPU and better cameras won’t entice would-be buyers enough to take the plunge.

Quite the contrary, the iPad 3 is picking up considerable steam even if much of its growth in the United States comes from owners of the original iPad and iPad 2 upgrading to the third-generation model…

According to a blog post over at Localytics, two months in and the new iPad has overtaken the original model in usage share.

Specifically, the third-generation iPad now claims a cool one-fifth of the market for all iPads in the United States, or 20 percent. That’s a 50 percent increase over the 14 percent share the analytics firm recorded in the first four days of iPad 3 availability.

Bear in mind that there were only four million units of the new iPad on the market during the launch week – and that includes nine other markets, not just the United States.

The iPad 2 remains in the lead with three times higher usage share versus the new iPad.

Localytics observes:

While its swift overtaking of the original iPad is impressive, it has a long way to go to leapfrog the iPad 2, which is still available in 16GB capacity for $100 less than the third-generation iPad’s basic model.

It’s worth pointing out that Localiytics derives these numbers from third-party apps using its analytics solutions.

Therefore, their survey may not be an accurate representative of the overall market, but it certainly highlights key trends.

During the iPad 3 launch week, the original iPad had a 24 percent share and the iPad 2 recorded a 62 percent share.

Since then, the iPad 2 has lost two percentage points and the original iPad has slipped by four percentage points.

To me, there’s only one conclusion: literally all of the iPad 3 growth in the United States has come from iPad 1 and 2 owners upgrading their devices.

If true, that has got to be a worrying sign.

Maybe I’m reading too much into these numbers?