If you send out email newsletters, or other email-based marketing, you may be interested in these stats from Campaign Monitor. They took a look at over 250 million opens over a 6 month time period and posted the results.
There is some fine print in the accuracy of the results:
“The email client a person is using can only be detected if images are displayed. This can give an inflated weighting to email clients that display images by default, such as Outlook 2000 and the iPhone. It will also provide a lesser weighting to those that block images by default such as Gmail and Outlook 2007. Those email clients that aren’t capable of displaying images, such as older Blackberry models and other mobile devices cannot be included in this study.”