Not that this helps the original poster, but Slicehost is pretty much pay-as-you-go as well now that they don't charge the first 3 months up front. They still charge a full month at a time, but if for example you create a slice for just a couple minutes, they automatically pro-rate it and refund the unused time.
Coupled with their API that lets you instantiate new slices just like EC2 instances, they're definitely the closest thing to EC2 I've seen going.
An EC2 pay-as-you-go model (opposed to a Slicehost one) might be a more economical option for testing as well.