It's been posted before, but this offer is still on. Just booked myself London Victoria to Brighton on 21st September for £3 each, and coming back on 23rd September also for £3.
It's shown on the National Rail website, but I had a great deal of trouble using their internet booking service, so I'd recommend the National Express East Coast website as it worked best for me. They can post the tickets to you at no extra charge, and no booking fee.
Use London Victoria as the departure station rather than London (All) to find the cheapest fares. This offer is valid on Southern Trains only, and according to their website you can even use some Railcard discounts. Further information here:http://www.southernrailway.com/main.php?page_id=499