bmossman, while Mark's link IS some good reading, here's some pertinent info for you regarding LDT.
You don't mention a version, but I suspect you are using 2008 with no Service Pack(s) installed. That is the only LDT release that I'm aware of that had this problem. Grabbing
SP2 and isntalling it should correct this. Although I don't think it will fix any labels already in the drawing (but it might).