Don't forget bandwidth costs. I could get about 100mbps to an external server. At $0.12/GB, 100mbps * 35 minutes = 25GB = $3. Most clicks won't cost you this much, but you should definitely keep an eye on this. Maybe set a bandwidth cap somehow.
Also, I noticed a public key in ~/.ssh/authorized_keys. Is that intentional or just part of how EC2 APIs work?
Other than that, great job. I would definitely use this service.