Landing at Dallas one day in an MD80 noticed the DME vs full down glide slope didn't make sense with FO flying so told him to level off. Soon after level off we were advised of a low altitude alert by controller. We were in the clouds around 1500 ft AGL. DFW ceiling was reported at 1,000 ft. A B747 was stopped next to the GS antenna causing the problem but tower said they didn't have to protect the sterile approach area until the ceiling got below 800 ft. Isn't that nice? The GS suddenly popped to full up so beware.