Here in most areas of New Hampshire, our power company's are fairly aggresive in keeping the power line ROW's clear of anything that could damage the lines and cause a outage. They use a machine very much like MarkV described. It is usually a large excavator with extra screens, grills, cages etc. to protect the operator and on the end of the boom is a device which is kind of best described as a big grinding attachment. there is very little if anything that it won't and can't shred.
The company name that I see, is usually the same, unfortunatly I can't think of it. But I have been told that the owner of the company is the guy who invented the particular type attachment the he uses.
I have had the chance to watch it work and it is rather amazing. I am doing some web searching to see if I can find anything on it or something similar, if I have any luck, I will let you know.