The software that was proposed to be closed source are portions of the online backup drivers. Each such driver has to be written in close cooperation with the developers of each storage engine. Well...
InnoDB already has an online backup tool, and even if/when they revise their tool to use this new API, it's still going to be theirs, open or closed, not the property of the MySQL Group.
Online backup of the engines for Archive, CSV, Blackhole, and Memcached doesn't even make sense, and even if it did, BrianA will flat out refuse to write crippleware into his own software.
Similarly, while online backup makes sense for Maria, I don't see MontyW writing crippleware into his work.
How about MyISAM? I think that work is already done, but, the horse is already out of the barn, in that the online backup drivers for it just went up on bkbits.
Looking even closer, the part that was going to be closed was not even the entire online backup driver set, but just compression and encryption. Any halfway competent developer would be able to hook in the necessary calls to azio, zlib, and openssl, and replicate the work.
So this is a big tempest over something that's not going to happen, and doesn't matter anyway.
Plus, best practices for backup dont even use or want online backup. The Right Way to backup a real production MySQL instances is via filesystem snapshot, using something like LVM or ZFS.
As a small aside, the Slashdot headline was not entirely accurate. It wasn't the Sun executives who decided this. It was the MySQL executives. What that means, especially in light of the keynote speeches given by CEO Jonathan Schwartz and VP Rich Green, is interesting, and remains to be publically seen.