According to email deliverability company Return Path:

We spend more time using our phones to read email, take pictures and update our Facebook accounts then we do actually using it to talk. And the growth of smartphones doesn’t seem to be subsiding with half of all Americans now owning a smartphone. We looked at nearly one billion data points for this mobile study to see how this growth was affecting email, and the numbers were clear: if you don’t know how many of your subscribers are reading your emails on a mobile device and you don’t have a mobile email strategy, you’re about to miss out on the next gold rush.

Our study found:

• Where and when subscribers are interacting with your email
• Year-over-year (March '11 – March '12), Email opens on mobile devices grew 82.4%, and is on track to surpass both desktop and webmail email views by midyear.
• Apple devices account for 85% of all mobile email opens.
• Time of day influences which device people read emails on with desktop email clients being the most popular during the weekday and webmail and mobile being more popular during the weekend.

See Return Path's infographic for more information.