There has been a lot of speculation as to who sent the trollocs, but I'm not particularily concerned with that. I'm more concerned with where they are and why.
At first glance Ebou Dar and the Seanchan seems like a counter productive target for the trollocs. The Seanchan believe trollocs to be imaginary boogie men used to scare kids. Why would the Shadow want to A) blow the surprise. I don't think the force is very large, certainly not on the scale of the 100K that attacked Rand. I would think that it would only anger the Seanchan and strengthen their resolve, not sow chaos and despair. B) more importantly I think it very likely that the early use of trollocs could force the Seanchan towards the Westlanders. At least the Westlanders are human!
I also don't think the Shadow is that dumb. I think there was a specific objective ie Tylee and her force. It is certainly concieveable that there are Shadow spies in her force. If so the Shadow probably knows of Tylee's admiration of Perrin and his abilities. If I were the Shadow I wouldn't want that kind of thing to spread, I'd want the two sides ready to tear out each other's throats. I'd try to make sure that Tylee and command structure didn't survive to make it back to Ebou Dar to be debriefed. Hence the trollocs. Get Tylee, get her commanders, and as many of her force as possible and you've eliminated a possible thorn in your side. The trollocs were specifically sent to ambush her force, kill her eradicate any kind of respect/admiration/good feelings towards the Westlanders.
So what do you think?

NaCl(that's my story and I'm sticking to it)H2O