gistfile1.txt

// 
// This is a signal that gets a users password. Imagine that it did something clever, like
// look in the keystore and prompt the user if the password isn't in the store
//
RACSignal *fetchPassword = [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {

  NSString *password = @"the password"; // imagine this required some async work to fetch
  [subscriber sendNext:password];
  [subscriber sendCompleted];
        
  return [RACDisposable disposableWithBlock:^{

  }];
}];


//
// Once we have fetched the password, we want to fetch a 
// list of items from a webservice that requires that password
//
[[fetchPassword flattenMap:^RACStream *(NSString *password) {
  return [AsyncClient fetchStuffListWithPassword:password];
}] flattenMap:^RACStream *(id stuffItem) {

  //
  // Now for each item that is in the list, we need to do
  // another API call, this one requires the original password
  // Ideally, I could pass that password in the flattenMap call
  //
  return [AsyncClient fetchStuffItem:stuffItem withPassword:password];
}];

Hm, that's actually not that simple! How about something like this (totally untested):

[[fetchPassword flattenMap:^id (NSString *password) {

    // I'm not sure whether this works, but the idea
    // is that the signal that returns the password
    // should just keep returning the password continuously
    // so that the -zipWith: doesn't terminate until there
    // are no more stuffItems.
    return [[[RACSignal return:password] never]
                       zipWith:[AsyncClient fetchStuffListWithPassword:password]]

}] flattenMap:^RACSignal *(RACTuple *passwordAndStuffItem) {

    RACTupleUnpack(NSString *password, id stuffItem);
    return [AsyncClient fetchStuffItem:stuffItem withPassword:password];

}];

Slight modification on your version. Basically I sub'd repeat for never (never is a class method in my version of RAC), and changed the unpack line a bit:

[[fetchPassword flattenMap:^id (NSString *password) {
    return [[[RACSignal return:password] repeat]
                   zipWith:[AsyncClient fetchStuffListWithPassword:password]]
}] flattenMap:^RACSignal *(RACTuple *passwordAndStuffItem) {
    RACTupleUnpack(NSString *password, id stuffItem) = passwordAndStuffItem;
    return [AsyncClient fetchStuffItem:stuffItem withPassword:password];
}];