@interface MyClass : NSObject

@property (readwrite, copy) NSArray *someArray;

@end

@implementation MyClass
{
    NSMutableArray *someArrayBacking;
}

- (id)init
{
    self = [super init];

    if (nil != self)
    {
        someArrayBacking = [[NSMutableArray alloc] init]
    }

    return self;
}

- (void)setSomeArray:(NSArray *)a
{
    @synchronized(self)
    {
        if (a != someArrayBacking)
        {
             [someArrayBacking release];
             someArrayBacking = [a mutableCopy];
        }
    }
}

- (NSArray *)someArray
{
     @synchronized(self)
     {
         return [[someArrayBacking copy] autorelease];
     }
}