This isn't a smart move by MySQL. Yes, you can get the sources yourself, but that means being au fait with BitKeeper (and having a client available), whereas tarballs are… well, *the* way of distributing sources. I'd also be surprised if the BK repository contained the pre-generated configure script (and related autotools magic), meaning you've got to have the right versions of autoconf, automake and libtool present and correct before you can think about building it.
“If you can't make your own tarball, what in the name of the Flying Spaghetti Monster do you think you're doing with an SQL database server?”
Developer != sysadmin != DBA. To build, install and run MySQL from source you'll need to be all three.
Still, if anything's going to push people towards PostgreSQL, this will. Shame, really; MySQL 5 is quite a good RDBMS.
Mind you, doesn't Debian post the upstream sources alongside the diff and dsc required to build a .deb? Assuming they continue to distribute MySQL, all of the debian sources mirrors will continue to carry MySQL sources, which is presumably how FreeBSD Ports, pkgsrc, MacPorts, et al, will continue to work.