iOS (jailbroken): Popular jailbreak tweak AutoResponder has updated for iOS 6, letting you auto-reply to SMS messages when you're away or busy. It's also available for $1.50, 50 per cent off its usual price, for a limited time.
AutoResponder is a simple tweak that shows up in your Settings app once it's installed. You can turn it on, set however long a delay you want, and create different automatic replies for things like when you're out of the office, driving, in a movie and so on. You can also whitelist certain contacts to whom you don't want to auto-reply.
A similar feature is available in iOS 6, but AutoResponder gives you quite a bit more control. Head on over to Cydia to download the tweak at half off while you can.