Eric,
The event you are looking for is the OnAfterChange event for the repeating node. To access it, double-click the node in the Data Source task pane, go to Data Validation and Event Handlers, select OnAfterChange from the drop-down menu and click Edit Code.
It's not quite like an OnClick event, because you have to do filtering sometimes, but this should do what you are looking for.
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge