Check it:

- (IBAction)toggleFavorite {
    DataManager *data = [DataManager sharedDataManager];
    NSMutableSet *favorites = data.favorites;

    if (thisEvent.isFavorite == YES) {
        NSLog(@"Toggling off");
        thisEvent.isFavorite = NO;
        [favorites removeObject:thisEvent.guid];
        [favoriteIcon setImage:[UIImage imageNamed:@"notFavorite.png"] forState:UIControlStateNormal];
    }
    else {
        NSLog(@"Toggling on, adding %@", thisEvent.guid);
        thisEvent.isFavorite = YES;
        [favorites addObject:thisEvent.guid];
        [favoriteIcon setImage:[UIImage imageNamed:@"isFavorite.png"] forState:UIControlStateNormal];
    }
    NSLog(@"favorites array now contains %d members", [favorites count]);
}

This is fired from a custom UIButton. The UI part works great--toggles the image used for the button, and I can see from other stuff that the thisEvent.isFavorite BOOL is toggling happily. I can also see in the debugger that I'm getting my DataManager singleton.

But here's my NSLog:

2010-05-13 08:24:32.946 MyApp[924:207] Toggling on, adding 05db685f65e2
2010-05-13 08:24:32.947 MyApp[924:207] favorites array now contains 0 members
2010-05-13 08:24:33.666 MyApp[924:207] Toggling off
2010-05-13 08:24:33.666 MyApp[924:207] favorites array now contains 0 members
2010-05-13 08:24:34.060 MyApp[924:207] Toggling on, adding 05db685f65e2
2010-05-13 08:24:34.061 MyApp[924:207] favorites array now contains 0 members
2010-05-13 08:24:34.296 MyApp[924:207] Toggling off
2010-05-13 08:24:34.297 MyApp[924:207] favorites array now contains 0 members

Worst part is, this USED to work, and I don't know what I did to break it.