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
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.