You can now purchase reserved instances for applications that run on a part-time basis. From Amazon:
You can now purchase Amazon EC2 Scheduled Reserved Instances for applications that run on a part-time basis. Reserved Instances provide a capacity reservation so that you can have confidence in your ability to launch the number of instances you have reserved when you need them.
Starting today, the new Scheduled Reserved Instances option allows you to reserve capacity on recurring daily, weekly, and monthly schedules. For example, you can purchase a daily reservation such as midnight to 6am every day, a weekly reservation such as 8am-5pm every weekday, or a monthly reservation such as the first five days of each month.