You can customize the Cancel button on iOS 5 by using the appearance proxy.
You need to change appearance of UIBarButtonItem when contained in UISearchBar.
For example to change the title font of the Cancel button you can use:
NSDictionary *attributes =
    [NSDictionary dictionaryWithObjectsAndKeys:
     [UIColor whiteColor], UITextAttributeTextColor,
     [UIColor colorWithRed:0 green:0 blue:0 alpha:0.5], UITextAttributeTextShadowColor,
     [NSValue valueWithUIOffset:UIOffsetMake(0, 1)], UITextAttributeTextShadowOffset,
     [UIFont systemFontOfSize:12], UITextAttributeFont,
     nil];
[[UIBarButtonItem appearanceWhenContainedIn:[UISearchBar class], nil]
    setTitleTextAttributes:attributes forState:UIControlStateNormal];
[[UIBarButtonItem appearanceWhenContainedIn:[UISearchBar class], nil]
    setTitleTextAttributes:attributes forState:UIControlStateHighlighted];