EC2 Reserved Instance Billing Gotcha

So you purchased an EC2 Reserved Instance and you're looking to see the reduced hourly rate take effect immediately.

Don't.

Unlike all the other resources in EC2, which you can use immediately, reserved instances are not actually available until the credit card charge for your account is processed.

You'll need to wait until the next billing period begins the charge clears your credit card before the reserved instance is actually available and the discounted hourly rate takes effect. Until your credit card charge goes through, your instances will continue to be billed at the regular hourly rates.

It's a classic gotcha, but it's in the EC2 FAQ.

Update: According to AWS folks, the charge for the reservation is posted to your credit card soon after you order the reserved instance, instead of waiting for the next billing period. Here is the AWS Forum thread where AWS says so.