In most cases you can trim vegitation back to the property line as long as you don't damage the tree. If the tree becomes unbalanced because you cut it and it fell you could be liable. It can be a problem but if you can work with your neighbor to keep the tree healty and sized it is the best. If you can't work with the neighbor to maintain it, write them a letter and ask for a professional opinion on the safety and health of the tree. YOu can offer to pay a portion of the cost or just insist that the owner of the property take care of it. The person on whose land the tree or vegitation is planted is responsible for it.