Do you remember Broderick Bode, the Unspeakable that was injured in a confrontation with some Death Eaters in the ministry and was no longer able to speak. He was sent to St. Mungo's, and when he started to recover from the spell, he was killed by a cutting of Devil's Snare.
When the trio found out about the murder, they had an exceptionally long talk about killing somebody by using a Devil's Snare, and even Hermione was surprised about such a subtle kind of murder. This kind of left me with the impression that this murder is of greater importance than it seemed at first. I wondered:
A Devil's Snare is a rather dangerous plant, and putting a cutting of it in a pott to send it to a hospital, btw an environment a Devil's Snare doesn't like at all, certainly requires some herbiology skills. Who of the Death Eaters could do such a thing? It seems as if only the highest ranking DEs were involved in the plan to steal the prophecy, and I really cannot think of Lucius Malfoy, the Lestranges or the Ministry's executioner as people with a special interest in plants. So who sent it?
In the first book we learn that Mrs Sprout has some experiences with Devil's Snares, she used one to protect the Sourcerer's Stone, so I think it points out to her.
Now, I'm certainly not suggesting that Sprout is in fact a Death Eater, but she could still have acted under Imperatus, or maybe someone else stole the plant. Whichever way, if It's true, there must be a traitor in Hogwarts.
I know that this seems kinda far-fetched, but I think that so much emphasis was put on that murder that it simply has to be a hint. As with the polyjuice potion, JKR often introduces something that later on becomes very important...sorry for the long post!
I kinda forgot
(sorry, I'm feeling kinda sarcastiv today).
No one put the Imperius Curse on Neville did they, who visits there every week, did they? 
May not be the case...just a thought...
, it's possible that Sprout is (or will be) under the Imperius Curse by Lucius in order to put the Devil's Snare at St. Mungo's (or tell whoever put it there about the plant itself). Although, I would have thought someone would have caught on to that at Hogwarts if that were the case, but the fake Moody managed to slip in undetected. With all these curses and polyjuice potions and switching spells, who KNOWS who is who anymore?