Stuns definitely work. I have helped a couple groups on LK10 normal kills since 4.0.3 and 4.0.3a, and all the usual stuns work. I'm not 100% sure but I'm pretty sure the charge/intercept stuns work, but sometimes it refuses to let you use the ability at all on valks -- probably a glitch. But when I did it on my warrior (as prot), Conc blow and Shockwave both worked as expected. I've seen other stuns work as well.
Note though, that snares (like roots) shouldn't have any effect; only slowing effects and stuns work.