Visited 15.5.2010, after a walk up Fan Fawr. The name of this place appears to translate as "Ridge (of) the old slope"?
Some directions from Fan Fawr - the stone isn't marked on the OS 1:25000. I started from the trig point at the south-west end of the summit. From here, walk NE towards the summit. You will pass the top of a cwm dropping away to the south - go past this until you come to another cwm, heading SE off the summit ridge. Follow this SE until you come across a small stream, then follow the stream, keeping to the west side of it. You will be losing height steadily as you go. After a few hundred yards, the stream meets another coming in from the north - the stone is lying flat above and to the west of the point where the two streams meet. It's the only sizeable stone anywhere nearby, which does give some credence to the fact that it may be deliberately positioned here.
It's a fair sized slab, over 2m long, coming to a nice point at the north end (or the top, if it really has fallen over). If stood on end, it would have resembled a smaller Maen Llia. The alignment where it lies is roughly north-south, although it's actually a couple of degrees west of magnetic north. The tops of Corn Du and Pen y Fan can just be seen, and there are extensive views to the south. Its position near the confluence of the two streams could perhaps be significant? Coincidentally (?) it is also on the boundary between Glyn Tarell and Ystradfellte parishes, so it may have been used as some kind of territorial boundary marker for a very long time. One other thing to mention is the altitude - at 610m, this is higher than Maen Llwyd, so if it ever was a standing stone I think that would have made it the highest one above sea-level anywhere in Wales.
I don't really know what to make of this in truth, although it's a great spot to come anyway.
"A possible fallen standing stone is located on a small ledge above, and to the west of, a stream. It is a large quadrangular boulder 2.6m long, 1.8m wide (max), 1.1m wide (min) and 0.7m thick.
The alignment of the long axis of the stone is north-south. No evidence for a setting could be found. The stone is surrounded by a shallow ditch, about 0.2m deep and 0.5m wide, probably the result of peat shrinkage. Small boulders in this ditch may be the remains of packing material or, more likely, derived from the stone through erosion or attempts at quarrying it."