As of August 2013, 47% of emails are opened in mobile devices, says Litmus

According to this article from Litmus:

Mobile opens hit a plateau from May through July, but August saw a 3% leap and mobile opens now represent 47% of market share—a new record high, and nearly a 24% change since a year ago. In response, desktop and webmail clients have dipped slightly; with desktop opens at 32% and webmail at 21%.