Replace UIBarButtonItem with UIActivityIndicatorView

Just create two different UIBarButtonItems

One for the activity indicator and another for a normal UIBarButtonItem.

UIActivityIndicatorView * activityView = [[UIActivityIndicatorView alloc] initWithFrame:CGRectMake(0, 0, 25, 25)];
[activityView sizeToFit];
[activityView setAutoresizingMask:(UIViewAutoresizingFlexibleLeftMargin | UIViewAutoresizingFlexibleRightMargin | UIViewAutoresizingFlexibleTopMargin | UIViewAutoresizingFlexibleBottomMargin)];
UIBarButtonItem *loadingView = [[UIBarButtonItem alloc] initWithCustomView:activityView];
[self.navigationItem setRightBarButtonItem:loadingView];
[loadingView release];
[activityView release];

UIBarButtonItem * normalButton = [[UIBarButtonItem alloc] initWithTitle...];
[self.navigationItem setRightBarButtonItem:normalButton];
[normalButton release];

When you want to switch them, just reassign the rightBarButtonItem to whichever.