You are correct about iOS. On my iPhone 4 with iOS 5.1, with remote images enabled (by default) it leaks 17 out of 32 categories. With remote images disabled, it leaks 0 out of 32 categories.
In contrast, Outlook for Mac 2011 leaks 2 out of 32 categories (audio and video) with remote images disabled. Mail.app 4.5 leaks 14 out of 32 categories with remote images disabled.
In contrast, Outlook for Mac 2011 leaks 2 out of 32 categories (audio and video) with remote images disabled. Mail.app 4.5 leaks 14 out of 32 categories with remote images disabled.