This has to be a bug. I just turned 50 on my scoundrel today, and I did not encounter this problem on any other planet. Mobs aggro from a much longer distance when stealthed. In fact, they don't aggro from the same distance when not stealthed. It doesn't make sense. It's difficult to play the way I've been playing up until 50, as stealth is mostly useless on Ilum for me right now. Hopefully they will identify this as a bug and fix it. Kind of disappointing to hit 50 and have this bug/problem.
Edit: Found this video when doing a search for this bug, trying to find others who may also be experiencing this problem:
http://www.youtube.com/watch?v=0x0i4_n_wRw