if the client does not authenticate using TLS with the OpenSSL libaray, it should be fine. if the server has an up-to-date version of OpenSSL the only problem that might arise is if it had been significantly comprimised at some time prior to the patch(which is not possible to detect,unfortunately). if no part uses OpenSSL at all, the issue doesn't affect Powdertoy.
Yay, someone actually reads xkcd.
Also, didn't he already do a comic about Heartbleed on Wednesday?
EDIT: Yup, http://xkcd.com/1353/
Our imaginations are safe!