The logic that controls which list item is selected is determined by the list's selectedIndex. When you click on an item, the list's selectedIndex is set to that item's index, which in turn puts the appropriate Repeated Item into it's "Selected" state (and puts the previously selected item into it's "Normal" state). This is all done for you by Flex, and you can't directly control the state of a Repeated Item (or, in fact, any other skinned component!). In order to set an item's state to "Normal", you need to set the list's selectedIndex to not be that item.
You can do this by creating an action sequence with a Set Property effect that sets the selectedIndex of the list to -1 (which means nothing is selected). Unfortunately, you can't do this from within the Repeated Item itself because the Interactions panel won't let you target objects that are up the hierarchy from the event source! So there's actually no good way to do what you want to do in the current version of Flash Catalyst. However, it's quite easy in Flash Builder:
So how do you do this on Flash Builder?
I'd also like to know how to get it done in Flash Builder.
I realise this has already been answered, but just incase anyone wants to do this in Flash Catalyst itself
All you have to do is select the data list, then go to the interactions panel.
After you have selected the state that it should transition to, Change the last drop down to "When a specific Item is selected", then choose which item links to what.
I hope this helps someone who doesn't have an idea how to use Flash Builder.