Still happens in the 1.6.2 snapshot as well
This most definitely is an issue in 1.6.1, I don't know why you put "Cannot reproduce."
On Windows 7, Minecraft 1.6.1, 64bit java 1.7, hard difficulty, superflat at y=4, slimes do not spawn.
So it's intentional?
Loading the same world in 1.5.2 spawns slimes instantly, so this is a new issue.