The following code posts to twitter:

NSString *compoundLoginString = [NSString stringWithFormat:@"http://%@:%@@twitter.com/statuses/update.xml",extractedUsername, extractedPassword];

NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:
[NSURL URLWithString:compoundLoginString] 
cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:20.0];

// The text to post
NSString *msg = tweetText;

[request setHTTPMethod:@"POST"];
[request setHTTPBody:[[NSString stringWithFormat:@"status=%@", msg] 
dataUsingEncoding:NSASCIIStringEncoding]];

NSURLResponse *response;
NSError *error;

if ([NSURLConnection sendSynchronousRequest:request 
returningResponse:&response error:&error] != nil){
    [self postSuccessfulAlert];
}else{
    [self postNotSuccessfulAlert];
} 

I am curious as to how I could check if the username and password is correct before proceeding to the above piece of code.

I found the following code in a tutorial, but am unsure how I would implement or call this function.

- (void)connection:(NSURLConnection *)connection didReceiveAuthenticationChallenge:(NSURLAuthenticationChallenge *)challenge {

    if ([challenge previousFailureCount] == 0) {
        NSURLCredential *newCredential;
        newCredential=[NSURLCredential credentialWithUser:[self username]
                                                 password:[self password]
                                              persistence:NSURLCredentialPersistenceNone];
        [[challenge sender] useCredential:newCredential
               forAuthenticationChallenge:challenge];
    } else {
        [[challenge sender] cancelAuthenticationChallenge:challenge];
        // inform the user that the user name and password
        // in the preferences are incorrect
        NSLog(@"Invalid Username or Password");
    }

}

Any ideas?

Please note, I have taken snippets of code from both of the following tutorials.

http://iphonedevelopertips.com/networking/post-to-a-twitter-account-from-the-iphone.html

and

http://icodeblog.com/2009/07/09/integrating-twitter-into-your-applications/