im pretty sure that it is a bunch of small "balls" of solder that they heat up with (im assuming) a heatgun, and then press onto the pcb.
surface mounting for chips.
they put chipsets and video card chips on this way...
im pretty sure that is what it is...but dont hold me to it.
HonestJohn is exactly right. Look at the memory on a GF4Ti or 8500 compared to the memory on any earlier cards if you want the difference. The 8500 and GF4Ti use ball-grid. Kingmax has been using ball-grid for quite some time now, too.
There are several applications other than memory that have been using ball-grid, as mentioned.