I've just started to work deadlifts in to my routine. I'm concerned because as I pull the weight up, my back arches considerably. How dangerous is this? Should I just lessen the weights for now?
Bad form from the beginning teaches bad form throughout. Drop the weights and do it correctly. You could also do sumo pulls to get the groove. I have been warming up sumo style before doing regular deads. I think it helps.
Yes, it is very dangerous to arch your back. You could herniate your spinal discs. As with any athletic movement, it is best to leave the ego at the door and start with a low enough weight which permits you to learn proper form. I had to unlearn poor lifting techniques a couple years ago and I can tell you that it is much easier to learn it right from the beginning. Also from a training perspective, using proper form will ultimately allow you to lift much heavier weights.