MWPersistableAttributedString.m

@interface MWPersistableAttributedString : NSMutableAttributedString <NSCoding>
@end

@implementation MWPersistableAttributedString

- (void)encodeWithCoder:(NSCoder *)aCoder
{
    [aCoder encodeObject:[self string] forKey:@"string"];
    
    NSMutableArray *attributes = [NSMutableArray array];
    
    [self enumerateAttributesInRange:NSMakeRange(0, [self length]) 
                             options:0 
                          usingBlock:^(NSDictionary *attrs, NSRange range, BOOL *stop) 
    {
        // This does nothing, nothing to see here....
        
        NSLog(@"range %@", NSStringFromRange(range));
        NSLog(@"attrs %@", attrs);
    }];
    
    [aCoder encodeObject:attributes forKey:@"attributes"];
}

- (id)initWithCoder:(NSCoder *)aDecoder
{
    if ((self = [self initWithString:[aDecoder decodeObjectForKey:@"string"]]))
    {
        for (NSDictionary *attributes in [aDecoder decodeObjectForKey:@"attributes"]) 
        {
            [self addAttributes:nil 
                          range:NSMakeRange(0, 0)];
        }
    }
    return self;
}

@end

usage.m

NSMutableAttributedString *mutText = [[[MWPersistableAttributedString alloc] initWithString:@"hello world"] autorelease];