iOS 4 will now put an exclamation badge on the Messages app as a way to inform you when an SMS text or MMS multimedia message fails to send.
Previously, the exclamation badge was only present inside the Messages app itself, beside the message that failed to send. Unless you were there and checking, you might not see it, especially not right away.
Now, if and when there's been a failure to send, as long as you pass by the Messages icon on your home screen, there's a good chance you'll see it.
For more on Apple's latest software, see our complete iOS 4 feature walkthrough.