Any latent build slave that interacts with a for-fee service, such as the EC2LatentBuildSlave, brings significant risks. As already identified, the configuraton will need access to account information that, if obtained by a criminal, can be used to charge services to your account. Also, bugs in the buildbot software may lead to unnecessary charges. In particular, if the master neglects to shut down an instance for some reason, a virtual machine may be running unnecessarily, charging against your account. Manual and/or automatic (e.g. nagios with a plugin using a library like boto) double-checking may be appropriate.
A comparitively trivial note is that currently if two instances try to attach to the same latent buildslave, it is likely that the system will become confused. This should not occur, unless, for instance, you configure a normal build slave to connect with the authentication of a latent buildbot. If the situation occurs, stop all attached instances and restart the master.