A new capability in System Center 2012 SP1 - Orchestrator allows an activity developer to define "cascading dependencies" between input parameters.  For example if the user chooses a state like 'California' in the State input parameter the City input parameter could be immediately updated to only allow the user to choose cities in California like Los Angeles, Sacramento, and San Francisco.  This code sample shows an example  of how to create a cascading dependency like this.