Every second in my log: "md5: BUG in libdispatch ..."
What's interesting is that the md5 process is getting spawned quite rapidly. You can tell that by the incrementing process id (pid) - 2257, 2258, 2259, 2260

Can you run top -u -s15 and see if you can determine the parent process of the md5?

From there, we can know what process is launching the chatty md5. Also, run a md5 process by hand md5 /dev/null to see if any error conditions are spit out other than the expected hash shown below:

Mac:~ me$ md5 /dev/null
MD5 (/dev/null) = d41d8cd98f00b204e9800998ecf8427e


Also, list any file synchronization tools you might have running - rsync, carbon copy cloner, dropbox, box.net, or similar... They often use md5 to tell if a file has really changed and needs to be synchronized.