BGA Via in pad
We're starting to see more an more .5mm ball pitch mid-count BGAs here at Screaming Circuits. When you've got 400 - 500 balls on a .5mm pitch BGA, routing becomes a real challenge. One of the solution designers are exploring involves putting vias directly in the ball land pad.
This can be a very helpful technique, both from the perspective of easing layout pain and with ultra high-frequency applications, can get your capacitors even closer to the chip.
If you are going to do this, we strongly recommend that you have your vias filled and plated over at your board house. From an assembly standpoint, this will result in the same reliability that you would expect without vias in pad.