The following are some common questions I've seen regarding MS beta exams. Please if you have any others.

Why should I listen to what you've written on this page?
Aside from being posted on the internet, thereby proving the accuracy and infallibility of everything here, you probably shouldn't. Email mcphelp@microsoft.com if you really want answers that are worth anything.
However, I have taken many Microsoft beta exams (12 as of now), so I'm not just making this stuff up. I also participated in 'cut score' meetings for two of the exams, which taught me a lot more about the process. Much of which I can't repeat or the chip implanted in my skull will explode.

How can I take a beta exam?
Microsoft sets prerequisites for beta exam candidates that vary by exam. In every case I know of, this has meant having passed a related exam or older version of the one going into beta testing. For exam 70-218, however, they excluded anyone who was already an MCSE on Windows 2000. They were trying to eliminate overqualified candidates. For some reason, Microsoft also requires you to subscribe to the 'MCP Flash' email newsletter in order to get an invitation

Wait a minute, if you have to have passed a previous exam to take a beta, how did they choose people for the first exam?
Long ago, MS beta exams were open to anyone interested. There were no prerequisites or invitations. But you had to pay half price to take the exam, so that encouraged candidates to screen themselves to a degree. There were also prizes for passing or sometimes just taking these beta exams - I got a bunch of exam vouchers and Visual Studio 6 Professional this way.

How do I know when a new exam is on its way?
Microsoft has a page that lists exams in development, it's worth checking once in a while. I also watch newsgroups related to certifications that interest me.

All my friends got invitations, but not me. What shall I do?
This happens to me all the time. Fortunately, my childhood fully prepared me for the emotional trauma of being an overlooked outcast. As with the kids playing kickball during recess, usually all you have to do to join is ask. Looking forlorn definitely won't work. Email mcphelp@microsoft.com and express your interest in the exam and any qualifications you think are relevant. I also include my MCP ID so they can check any criteria I hadn't thought of. They've always sent me the necessary information to register with very little hassle.

How long does an exam beta period last?
Usually about a week, but if an insufficient number of people take the test, Microsoft will extend the beta. They need a certain minimum to ensure the statistics gathered are valid. The first 4 or 5 betas I did all had significant extensions - I believe some lasted for months. But in the last couple of years, these extensions seem to be rare, so don't count on seeing one. 

How is a beta exam different from a live exam?
It's free. It's much longer than the live exam - up to twice as long. And you don't find out if you passed when you're done. Your result is snail-mailed to you anywhere from 5 to 10 weeks after the beta ends. If you passed the beta, you can usually see results posted to your Microsoft transcript a week or so before the mail arrives. When Microsoft announces a specific 'go live' date for an exam, it usually means that results will be on their way to beta testers soon.

If you pass a beta exam, do you still have to retake the exam after it goes live?
No, in fact you are prohibited from retaking an exam after passing it. Passing the beta counts for everything that passing the live version does.

Why would anyone want to take a beta exam?
Did I mention it was free? Many beta testers also enjoy participating in the process. It's likely that comments on questions during a beta are much more important than after the exam has gone live. There's also some competitiveness about being 'first' to pass an exam, and beta testers have a huge advantage there. There have also been serious problems with braindumps for these exams, eventually requiring great skepticism about results obtained after actual exam questions are widely available. While MS has recently taken some very positive steps in addressing this problem, avoiding any possible suspicion of cheating is one of the big factors that motivates me.

Why doesn't everyone try to take beta exams?
They are much longer than live exams. There is usually a lot less known about an exam during the beta. Is it considered easy or hard? How many questions are there? Are any particular topics emphasized? And there are probably no tools specifically designed to help prepare you for the exam during its beta period. No practice exams, no study guides, just you and your big giant brain. Also, since the beta periods are generally pretty brief and invitations are typically sent about one month in advance, some people are just unable to arrange the time to take the exam even without doing any specific preparation for it.

Is a beta exam easier or harder than the live exam?
In short, no. The question pool for the live exam will be composed of a subset of questions that appeared on the beta exam, just with a small number removed that proved to be extremely easy or difficult or had a strong negative correlation with overall exam performance. And those removed items would not be counted in the scores for the beta testers. The beta statistics provide, among other things, information about the relative difficulty of each question, so the live exams can be reliably structured to be exactly as difficult (or easy) as the beta exam is. Microsoft does not give beta testers any "bonus" or "penalty" for having taken the beta - their criteria for passing is the same in either case.
However the beta will be longer, so endurance may be a slight factor making it more difficult. And testers taking the live exam likely have many more resources available to help prepare for the exam.