2013.10.31追記
古いので参考にはならないと思います。
「iPhone&Androidアプリ内課金プログラミング完全ガイド」など参考にしてみてください。

エラーチェックも何も無い省略しまくりソフトの概略です。枝葉は自分でちゃんと付け加えてください。
＃コピペした時に漏れがあったみたいなので修正しました。2010.2.4

まずはStoreKitObserver

MyStoreKitObserver.h
#import <Foundation/Foundation.h>
#import <StoreKit/StoreKit.h>
@interface MyStoreKitObserver : NSObject <SKPaymentTransactionObserver> {
}
@end

MyStoreKitObserver.m
#import "MyStoreKitObserver.h"
@implementation MyStoreKitObserver
-(id)init {
	if (self = [super init]) {
	}
	return self;
}
- (void)paymentQueue:(SKPaymentQueue *)queue removedTransactions:(NSArray *)transactions {
}
- (void)paymentQueue:(SKPaymentQueue *)queue updatedTransactions:(NSArray *)transactions {
	for (SKPaymentTransaction *transaction in transactions) {
		switch (transaction.transactionState) {
		case SKPaymentTransactionStatePurchasing:
			break;
		case SKPaymentTransactionStatePurchased:
		case SKPaymentTransactionStateFailed:
		case SKPaymentTransactionStateRestored:
			[[SKPaymentQueue defaultQueue] finishTransaction:transaction];
			break;
		}
	}
}
@end

あとは適当にボタンを用意して

TestStoreViewController.h
#import <UIKit/UIKit.h>
#import <StoreKit/StoreKit.h>
@class MyStoreKitObserver;
@interface TestStoreViewController : UIViewController <SKProductsRequestDelegate> {
	MyStoreKitObserver	*mySKObserver;
}
-(IBAction)test1Action:(id)sender;
@end

TestStoreViewController.m
#import "TestStoreViewController.h"
#import "MyStoreKitObserver.h"
@implementation TestStoreViewController
-(IBAction)test1Action:(id)sender {
	SKProductsRequest *request= [[SKProductsRequest alloc] initWithProductIdentifiers:[NSSet
setWithObject:@"item1"]];	// 自分でつけたアイテムID
	request.delegate = self;
	[request start];
}
- (void)productsRequest:(SKProductsRequest *)request didReceiveResponse:(SKProductsResponse *)response {
	NSArray *myProduct = response.products;
	if([myProduct count] > 0) {	//1アイテムしか処理してません
		SKProduct *product = [myProduct objectAtIndex:0];
		SKPayment *payment = [SKPayment paymentWithProductIdentifier:product.productIdentifier];
		[[SKPaymentQueue defaultQueue] addPayment:payment];
	}
}
- (void)viewDidLoad {
	[super viewDidLoad];
	mySKObserver = [[MyStoreKitObserver alloc] init];
	[[SKPaymentQueue defaultQueue] addTransactionObserver: mySKObserver];
	UIApplication *app = [UIApplication sharedApplication];
	[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(_applicationWillTerminate:) name:UIApplicationWillTerminateNotification object:app];
	[[SKPaymentQueue defaultQueue] restoreCompletedTransactions];
}
-(void)_applicationWillTerminate:(NSNotification*)notification
{
	[[SKPaymentQueue defaultQueue] removeTransactionObserver: mySKObserver];
	[mySKObserver release];
}

一応動作確認はしましたが無保証です。