Solution found here: http://social.msdn.microsoft.com/forums/en-us/sqlreportingservices/thread/6E11EC25-E205-469E-8C35-B73B4478BCF0?prof=required
It's a JavaScript error that happens if one of the LABELS of a value of a multi-value parameter is a blank string. If you have any blank labels, change the labels to be something like "(blank)" and everything should be fine.