I got an email from Amazon last week, for the Windows instance:

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Hello,

It's come to our attention that one of your EC2 instances may be configured as a Tor exit node. Please note that any open proxy activity is prohibited in our AUP:

http://aws.amazon.com/aup/

Please respond to this email detailing the corrective measures you plan to take. If you believe our findings to be in error, please provide additional information about how this activity relates to your use case.

Regards,
Amazon EC2 Abuse Team