iMessage only works if both devices have iOS 5 (and if iMessage is turned ON). It won't work with a non-iOS device or if Apple's iMessage servers are unavailable.
And if all else fails, a soft reset to the device could fix the issue at hand.
Press and hold the Home button and the Sleep button at the same time for at least ten seconds, until the Apple logo appears. None of your apps or settings will be lost. Sometimes it's necessary if a buggy app crashes.