The tree is now public:
http://mysql.bkbits.net:8080/mysql-5.2-falcon
Instructions can be found here on how to download and compile from the source trees:
http://dev.mysql.com/doc/refman/5.1/en/installing-source-tree.html
Just to point this out:
1) Don't use this in production.
2) Don't assume you can upgrade from it
3) While we haven't designed it to crash, this is compiling from a source tree so bugs are very likely (please report bugs to bugs.mysql.com)
The changeset that is likely to become the Alpha is "Cset 1.2384"
Have fun!