Yup, it is pretty new (a little over a week) and we're testing a few things with it. I collaborate with a lot of other very smart forum owners... many who have much larger and many more forums, and they suggested we give this a try to increase the conversion of guests to members. Most people come to BYC from Search Engine results and don't even hit the homepage, so we're hoping that having it on all pages will really drive the benefits of joining BYC.
There is also a new advertisement placement along with the "join us banner" which is helping to allow us to add new features and make upgrades like
this one, which was added after we put up the new ad and "join us" banner. It is definitely a bit more prominent, but we still pride ourselves for having way fewer ads than most large websites (and even newspapers, magazines, and TV for that matter) and also better quality ads.
Regarding registrations: It is interesting that BYC is often people's first experience with an online forum, so "shouting" to them the benefits of joining, with a bit "JOIN NOW" button can make a huge difference. So far the results have been fantastic and we're seeing more people join than ever before!
Again, while we recognize that it may annoying to some that aren't logged in, we hope the pros outweigh the cons (so far the tests look very positive) and as Terrie mentioned, it is really easy to skip them... just login.
Oh, and on a related note: for the first time EVER we had a whole month of daily registrations on BYC that were each over 100! We've been really close before, but there was always a day (usually a Thursday, Friday, or Saturday) where we dipped below 100.