Microsoft has just released KB 2850370:
Modifying a Configuration Manager 2007 service window may cause an unexpected delay in a software update deployment
Consider the following scenario:
• You create a maintenance window for a Microsoft System Center Configuration Manager 2007 collection.
• You create a new software update deployment with a deadline that is prior to the maintenance window of the System Center Configuration Manager 2007 collection.
• You modify any property of the maintenance window after the software update deployment deadline.
In this scenario, this may result in a missed opportunity to install updates right after the new service window becomes available. This happens because the Update Deployment Agent (UDA) finds no suitable service window for the installation, thus the new service window targeted to the client is ignored until the next evaluation cycle.
You may also see the following message in updatedeployment.log:
OnServiceWindowAvailable – No pending install assignment
This is by design in System Center Configuration Manager 2007.