As per a recent post to the EC2 forums, there's now support for reserved instances inside the EC2 EU Data Center:
Dear AWS Customer,
We are excited to announce that Amazon Elastic Compute Cloud (Amazon EC2) Reserved Instances are now available in Europe. Reserved Instances give you the option to make a low, one-time payment for each instance you want to reserve and in turn receive a significant discount on the hourly usage charge for that instance. After the one-time payment for an instance, that instance is reserved for you, and you have no further obligation; you may choose to run that instance for the discounted usage rate for the duration of your term, or if and when you do not use the instance, you will not pay usage charges on it.
Please visit http://aws.amazon.com/ec2 for more information on Reserved Instances, including pricing for the Europe Region.
The Amazon EC2 Team