Open beta =/= Stress test. Open beta is a beta test without a NDA, usually designed to test the software stability. The stress test is designed to test the hardware and network. Although over the past few years, many companies have been doing the stress test and the open beta at the same time, which is why many people think open beta = stress test/open to everyone.
At this point, nobody knows if it's open to everyone or if it requires a key. Although most stress tests only go on for a few days, not a few weeks. Because of that, I'm going to guess that it won't be completely open. But that's just a guess.