Committed on:
Portal 6.1.x CE GIT ID: ae054c25efe0c6a2c8e770c3e088503079acc678.
Portal 6.2.x GIT ID: e7447dbb56b17d57b56e9611f41cf9b48ece0da5.

Hi Matthew:
I moved this ticket to "community backlog", meaning that his feature is interesting and can be implemented by the community or Liferay as an improvement for a future version.
We will appreciate if you or any community member contribute this functionality
Thanks for your feedback!

Something along the lines of

Attribute attribute = attributes.get(id);
int size = attribute.size();
String value;
if(size > 1) {
StringBuilder sb = new StringBuilder(attribute.get(0).toString());
for (int i = 1; i < size; i++) {
sb.append(",").append(attribute.get:info.toString());
}
value = sb.toString();
}