We had a requirement wherein we wanted to update one of the fields in the incident record when we are resolving it through (Actionà Resolve Case).
Here we had to write a Pre-Update plugin registered on Child Pipeline against incident entity.
More on that
http://roman20007.wordpress.com/2010/04/25/resolve-case-event-in-plugin/
http://social.microsoft.com/Forums/en-HK/crmdevelopment/thread/ef1e594f-f50b-484e-a166-285429df8e4d
Hope it helps.