LoadGen.java

 public Iterator<Transaction> iterateData() throws Throwable {

        final InputData inputData = new DataLoader().loadData();
        final SeedFactory seedFactory = new SeedFactory(seed);

        System.out.println("Generating stores...");
        final ArrayList<Store> stores = new ArrayList<Store>();
        final StoreGenerator storeGenerator = new StoreGenerator(inputData, seedFactory);
        for (int i = 0; i < nStores; i++) {
            Store store = storeGenerator.generate();
            stores.add(store);
        }

        final List<Customer> customers = Lists.newArrayList();
        final CustomerGenerator custGen = new CustomerGenerator(inputData, stores, seedFactory);
        for (int i = 0; i < nCustomers; i++) {
            Customer customer = custGen.generate();
            customers.add(customer);
        }
        Long nextSeed = seedFactory.getNextSeed();

        Iterator<Transaction> transactions = new Iterator<Transaction>() {
            long index = 0;
            Iterator<Customer> iter = customers.iterator();

            @Override
            public void remove() {
                throw new RuntimeException("unsupported");
            }

            @Override
            public boolean hasNext() {
                if(index%1000==0){
                    System.out.println("Transaction: " + index);
                }
                return true;
            }

            @Override
            public Transaction next() {
                try {
                    index++;
                    Iterator<Customer> custIter = customers.iterator();
                    Collection<ProductCategory> products = inputData.getProductCategories();
                    Customer cust = custIter.next();
                    //Create a new purchasing profile.
                    PurchasingProfileGenerator profileGen = new PurchasingProfileGenerator(products, seedFactory);
                    PurchasingProfile profile = profileGen.generate();
                    TransactionGenerator transGen = new TransactionGenerator(cust, profile, stores, products, seedFactory);
                    List<Transaction> transactions = Lists.newArrayList();
                    return transGen.generate();
                } catch (Throwable t) {
                    t.printStackTrace();
                    throw new RuntimeException("See above failure.  failed creating next transaction due to  " + t.getMessage() + " " + t);
                }
            }
        };
        /**
         * Return the iterator.
         */
        return transactions;
    }

Awesome start to a streaming driver. :)

For correctness, you should only create one PurchasingProfile and one TransactionGenerator per customer. Maybe you should create a List as an instance member and populate it with one TransactionGenerator per customer?