There'a an app on Cydia called "iSchedule" which has this functionality, but it only works for SMS messages sent to phone numbers.

A lot of my contacts use their iPhone (softbank) email address for SMS messaging, which doesn't seem to be supported.

It would be so nice to be able to type my replies instantly and then choose a time for them to be sent. If anyone knows of something that can do this, let me know.

In before "just manually send them later". That's what I'm doing now, but I'd like to be able to fire-and-forget, as it were.