J.,
First of all, don't panic. This is normal (which is not to say acceptable) toddler behavior. Most of them go through a hitting/biting/hair pulling/whatever phase. I don't agree with the other E. who wrote that this is somehow your fault or even someone else's fault. Kids hit. They get frustrated and don't know what to do with it and they lash out. If TV or daycare is the cause of hitting, then I guess nobody hit anybody else until 50 years ago? Anyway, what is important is how your respond to this behavior, and if it has been 6 weeks it is high time to get on top of it.
I'm not sure what your discipline strategy is, but I think first you need to get on the same page as the daycare. My son's daycare's strategy is exactly what we do at home as well so he is given a clear, consistent message about what behaviors are unacceptable and what the consequences of such behaviors are. For most unacceptable behaviors, a warning is issued, and if it is not heeded then a consequence follows, usually either removal of the toy (or whatever is causing the behavior) or a time out. They call them "breaks" at my son's daycare, so that's what I tend to call them too, but they are really time-outs on the "naughty stair." For really unacceptable behaviors (hitting, doing something dangerous), an immediate time out, with a message of "You are taking a break because you hit. Hitting hurts. We don't hit our friends (Mom, dog, whomever)." 1 minute per year of age, and when it is time to get up you reinforce with "You had to take a break because you.... If you are ready to get up, you need to apologize to.... for...."
It does work, if you are calm, clear, and consistent. And by that I mean you may have to march him back to the naughty stair (or chair, or corner, or whatever you use) 40 times in the beginning, and it might take 2 hours-- whether you have 2 hours to spend on it or not. It is not something to start on a whim, because, like sleep training, if you cave, you will have created a MONSTER.
My ds is only 2, but at 3 I would think that you could be working with your son to help him verbalize what he is so angry/frustrated about and what other strategies he could use to resolve them. Mr. Rogers has a great song called "What do you do with the mad that you feel?" and a whole episode on anger. See if you can find it on DVD to help him work through these feelings.