We recorded it and did a slow-motion replay, it sure doesn't look accidental.
Evidence it was on purpose:
1. Justin Timberlake appears to deliberately reach across her chest, grab on, and pull the front of her dress off.
2. She's wearing a little tassle that covers her nipple and was invisible when her outfit was on.
Evidence that it was unexpected:
1. Janet looks down in what may be surprise after it happens.
2. Justin's facial expression immediately before the cut-away seems slightly startled.
My verdict:
It was on purpose. The trump card: Why would she wear a nipple tassle that was COMPLETELY INVISIBLE before her breastplate was pulled off? Not only that, but how do you accidentally reach across her chest, grab onto it, and rip it off? That would be quite a seizure.
yay
The real jerks here are CBS. It was obviously on purpose, grow a pair and stop kowtowing to the prudes that freaked out. Just because your core audience seems to be 'Touched by an Angel'/'Highway to Heaven'/'Joan of Arcadia'/Metamucil Variety Fun Time Hour doesn't mean you need to be so cowardly. Just once I'd like to see them say something like 'Yeah, we did it. Life is short, you might as well have fun once in a while. If anyone was offended, screw you, you know that you'll be back for more Dr. Quinn next week. What are you going to watch, wrestling?'