[2.0.410]

[teedev]

It is not possible to enter a "new Ext.XTemplate(...)" in the property itemTpl of a list component. The code is destroyed. The results differs if you enter to code in the property inspector or in the popup which pops up if you click the cog icon of a list. See:
http://www.sencha.com/forum/showthread.php?195495-How-to-use-List.itemTpl-(Sencha-Designer)&p=782665#post782665

[aconran]

Sencha Architect only supports strings in the template configuration at the moment.

In a version coming in the near future, you will be able to add a template node, get a full screen editor and add member formatting functions to that template.