Last week I was trying to change the work item type definition so it will set a default value in the iteration path. But when I tried importing it back with the TFS Power Tools, I always got the following error:
Error importing work item type definition:
TF26062:Rule '<DEFAULT from ="value" value="283" />' is not supported for the field 'System.IterationId'.
It seems like it is not possible to set a default value for the area or iteration path. In the end I solved this by creating a Work Item Template(part of the TFS Power Tools) with a default value for Iteration Path. I really like this approach because I can change the value in the template when a new sprint starts.
More about Work Item Templates and their usage: