Can I force an iPhone user to upgrade an application?

I have done this feature by getting version from itunes webservice and comparing with the current version.. Following is my code

        NSString *version = @"";
        NSURL *url = [NSURL URLWithString:@"http://itunes.apple.com/lookup?id=<Your app ID>"];
        versionRequest = [ASIFormDataRequest requestWithURL:url];
        [versionRequest setRequestMethod:@"GET"];
        [versionRequest setDelegate:self];
        [versionRequest setTimeOutSeconds:150];
        [versionRequest addRequestHeader:@"Content-Type" value:@"application/json"]; 
        [versionRequest startSynchronous];

        //Response string of our REST call
        NSString* jsonResponseString = [versionRequest responseString];

        NSDictionary *loginAuthenticationResponse = [jsonResponseString objectFromJSONString];

        NSArray *configData = [loginAuthenticationResponse valueForKey:@"results"];

        for (id config in configData) 
        {
            version = [config valueForKey:@"version"];
        }
   //Check your version with the version in app store
        if (![version isEqualToString:[itsUserDefaults objectForKey:@"version"]]) 
        {
            ProAlertView *createUserResponseAlert = [[ProAlertView alloc] initWithTitle:@"New Version!!" message: @"A new version of app is available to download" delegate:self cancelButtonTitle:@"Cancel" otherButtonTitles: @"Download", nil];
        [createUserResponseAlert show]; 
        [createUserResponseAlert release];
        }

And put the app id in the itunes link see the code below..

- (void)alertView:(UIAlertView *)actionSheet clickedButtonAtIndex:(NSInteger)buttonIndex {
    // the user clicked one of the OK/Cancel buttons
    if (buttonIndex == 1)
    {
        NSString *iTunesLink = @"itms-apps://phobos.apple.com/WebObjects/MZStore.woa/wa/viewSoftwareUpdate?id=<appid>&mt=8";
        [[UIApplication sharedApplication] openURL:[NSURL URLWithString:iTunesLink]];
    }
}

Note : You need JSONKit frame work and ASIHttp frame work to make the webservice calls..