There is this web page (I blogged about it a few months ago) where you get an interface to BitKeeper to watch the development activities: http://mysql.bkbits.net:8080/mysql-5.1/index.html.
Just for fun I replaced 5.1 with 5.2 and - it worked. So I followed the instructions from the manual and only replaced 5.1 with 5.2 again. This way, I ended up with a MySQL 5.2.0-alpha installation:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 3 to server version: 5.2.0-alpha
Type 'help;' or '\h' for help. Type '\c' to clear the buffer.
I doubt that there are many (if any at all) differences to the current 5.1 development source - but it's nice to see that MySQL 5.2 is on the way.
And I'm really excited to hear about the new features that are planned for 5.2!