I've just moved from a country where I had unlimited (really) data on my iPhone contract to one where data plans are metered and expensive. So this recent article by David Chartier, on the Finer Things in Tech web site, comes at the right time. It points out the simple setting in iOS to turn off automatic loading of images in Mail. As with Mail on OS X, you can load images later, but you won't need to load them for every message, saving download time and bandwidth.
To change this setting, go to Settings > Mail, Contacts & Calendars, and toggle Load Remote Images to OFF. If you get an email with images, and want to see them, just tap on Load All Images in the message.
This setting would make more sense if it only affected image downloads when using cellular data. But it's an all-or-nothing choice, so even when you connect via Wi-Fi, you'll need to download images manually, if you use this setting.
Mac OS X Hints