I just got off the phone with a very helpful representative from Hasbro. She explained the official release date for Star Wars/Transformers Attacktix and the Intergalactic Showdown Packs are "June." There is no specific date in June, though Star Wars fans are familiar with boxes stamped "Do not put on shelves until..."
It was also explained that stores like Wal-Mart get their shipments direct from the overseas manufacturer. There are three distribution hubs they are shipped to, and then sent out to YOUR store from there. These hubs should get their stock in June, and then they will ship to stores. Some stores may not receive them if they have an overstock of Series 3, or Starters. This differs by store, but explains why some Wal-Marts have them and others do not.
Ultimately, there is no way of knowing when your local stores will get them in, but the month is coming fast. Kay-Bee Toys usually gets a manifest the day before they trucks come in. On a slow day you may be able to get a manager to take a look. Toys R' Us also has a similar system, and I believe their computers can check more advanced dates. Once June hits, call around, or stop by your local stores and be sure to ask! If you get any information be sure to post it here!