Specifically, iPhone users saw the greatest growth in game-playing time across the past three month. That coincides with the launch of the iTunes App Store in early July, which brought games and other custom applications to the iPhone-owning masses.

Unsurprisingly, the study found that smartphones are still primarily used for communication purposes such as phone calls, e-mails and text messages, with 46% of those surveyed reportedly using their smartphones more now than 3 months ago.

In all, NPD reports that portable devices, such as the iPod, iPhone, PSP, Nintendo DS, and smartphones, saw the most usage in-home, suggesting that "these devices may be competing with more stationary entertainment devices for a user's time."

For a look at the report's methodology, check below.

Methodology

The survey was fielded to an individual pre-identified sample of owners of portable devices such as portable gaming systems, smartphones and PDMP. The data was collected from 3,258 members of NPD's online consumer panel. Fieldwork was conducted from September 16 through Tuesday, September 23, 2008.