AppDelegate.m

+ (AppDelegate *)sharedAppDelegate {
    return (AppDelegate *)[[UIApplication sharedApplication] delegate];
}

archive.m

[[[AppDelegate sharedAppDelegate] managedObjectContext] save:nil];
NSData *archivedObjects = [NSKeyedArchiver archivedDataWithRootObject:objects];

NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];
[userDefaults setObject:archivedObjects forKey:@"someKey"];
[userDefaults synchronize];

SSManagedObject.h

//
//  SSManagedObject.h
//  Archiving NSManagedObject with NSCoding
//
//  Created by Sam Soffes on 2/28/10.
//  Copyright 2010 Sam Soffes. All rights reserved.
//

@interface SSManagedObject : NSManagedObject <NSCoding> {
	
}

+ (NSManagedObjectContext *)mainContext;

@end

SSManagedObject.m

//
//  SSManagedObject.m
//  Archiving NSManagedObject with NSCoding
//
//  Created by Sam Soffes on 2/28/10.
//  Copyright 2010 Sam Soffes. All rights reserved.
//

#import "SSManagedObject.h"
#import "AppDelegate.h"

static NSString *const kURIRepresentationKey = @"URIRepresentation";

@implementation SSManagedObject

#pragma mark - Class Methods

+ (NSManagedObjectContext *)mainContext {
	AppDelegate *appDelegate = [AppDelegate sharedAppDelegate];
	return [appDelegate managedObjectContext];
}


#pragma mark - NSCoding

- (id)initWithCoder:(NSCoder *)decoder {
	NSManagedObjectContext *context = [[self class] mainContext];
	NSPersistentStoreCoordinator *psc = [context persistentStoreCoordinator];
	self = (SSManagedObject *)[[context objectWithID:[psc managedObjectIDForURIRepresentation:(NSURL *)[decoder decodeObjectForKey:kURIRepresentationKey]]] retain];
	return self;
}


- (void)encodeWithCoder:(NSCoder *)encoder {
    [encoder encodeObject:[[self objectID] URIRepresentation] forKey:kURIRepresentationKey];
}

@end

unarchive.m

NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];
NSData *objectsData = [userDefaults objectForKey:@"someKey"];
if ([objectsData length] > 0) {
    NSArray *objects = [NSKeyedUnarchiver unarchiveObjectWithData:objectsData]];
}

Very useful, Thanks!