A quick pulse effect on the line item being checked would signal that something has started as well. As it is just checking the box doesn't seem to do anything if you don't notice the loading. Visual feedback that you've started an operation as opposed to just changing local state is very important. The only reason I didn't go looking for an "Update" button was that you'd said it was AJAX. :-)

I have a big AJAX application that's going into production next week here to which I added fade effects and so on; at first people laughed and thought it was just a ridiculous developer video game mentality, but then they started to realize the importance of change indication. Additionally, gradual changes are much less jarring to the user that simply inserting or removing rows and the like. We have another app in development that doesn't do that, and the difference is quite apparent.