And still broken as of 13w09b, with the pre-release this week if this bug isn't noticed and fix there is a good chance it will make it into the official 1.5 release.
Still broken as of 13w09a. If this gets overlooked and makes it into the official release of 1.5 I will be really, annoyed.
@Manuel, yes I do think it's a bud related glitch.
@Charles, 1.5 has also changed timing in redstone circuits so some repeaters may need adjusted. My 2 wide secret stairs worked after I added 1 tick extra delay to two repeaters.
Still broken as of 13w7a
Mobs going to the nether is not a bug.
This is not a duplication of the piston glitch that causes a ghost bug. Vertical piston extenders seem to break every other piston
http://img.photobucket.com/albums/v209/Daray_Ryo/Minecraft_Mistakes/2013-01-31_125137.png
This has a greater then 50% failure rate.
http://img.photobucket.com/albums/v209/Daray_Ryo/Minecraft_Mistakes/2013-01-31_125203.png
I can confirm, this is working as of 13w09c. I was a little worried when the stable example was a horizontal extender but all of my vertical tests work fine. I did find a new piston deleting bug though, I will report that when I have time.