A decree of divorce, which included the division of an IRA account, was signed off by the judge but indicated that the divorce was not effective until a future date (a date which has not yet passed). Prior to the effective date of the divorce, can this be considered a valid document for the purpose of effectuating an IRA to IRA transfer incident to divorce? (This is not a decree nisi.)

Unlike a decree nisi, the divorce decree you describe will be self-effectuating on the future date (no need for a decree absolute or any other future document--just the passage of time and voila, they'll be divorced).

Prior to that specified future date is this divorce decree incidental to divorce, something that will permit an transfer incident to a divorce?

I don't think so. The judge could for some reason perhaps revoke or amend it in the meantime in such a way that there would be no divorce for which this decree would be incidental. Before the future date specified for the divorce to take effect, I think it would be premature to act on it in effecting the transfer.

I would certainly provide a copy to the IRA custodian so that it is fully aware of the interest to be transferred, so that the IRA custodian is put on notice and does not allow the IRA owner to withdraw from the IRA thwarting the later transfer of the decreed amount.

Closest authorities I know of on this are PLRs 9344027 and 9006066.