Recently, I tried out the Microsoft Outlook for iPad app. One of the first things I noticed was that remote images are automatically displayed. There is no way to turn this off. While most people wouldn't notice this, the impact is that spammers will have the ability to determine if/when you've opened the email, as well as perform other analytics. Spear phishers have the opportunity to inject malicious images into email messages.
Overall, while the risk posed isn't very high, this does have the potential to cause an increase in spam. Because of this, security-conscious companies should consider blocking this app from their environments.