"You can earn a sustainable middle-class living doing iPhone apps that have some real substance to them," Grigsby said.
But let's look at the numbers. Kardell's app got around 10,000 purchases at $0.99 to $1.99 each, of which he gets 70%, so let's say $14,000. Entry-level salary for a software engineer in Minneapolis is $46-65K, so he needs at least 4 equally popular hits per year in order to come out ahead of an office job. The median return on an app is probably going to be a lot less--- even if Apple doesn't deny you access to their marketplace for arbitrary reasons (or shut you down retroactively and demand that you give your customers refunds.)
Does that mean there really isn't any money in app development? No, but the article makes it fairly clear that full-time employment consists of building apps for other companies.
[Grigsby] pointed to a Bloomington-based company called Nerdery Interactive Labs, a veritable factory of 70 developers who pair with clients who have an idea for an app but lack the technical know-how to create one. Its uncredited work includes apps for Delta Faucet and J.C. Penney.