Vias in between BGA Pads
You might want to look into the 'Dog Bone' BGA escape pattern. Basically, the BGA is split into 4 quadrants. Each ball has a single via placed towards the outside of it's respective quadrant. It makes it very easy to route out signals this way, and is the de-facto standard in many designs. If you have a signal heavy design, it's imperative that you can run traces in between the ball and the via - this can cost big $$ on smaller pitch BGAs! 

The key thing is to make sure each via is covered in solder mask, otherwise you could potentially get bridging and other DFM hiccups. You also have to pay attention to the size of the trace connecting the ball pad to via - too big is sometimes not good, due to the shrinkage of solder mask on ball pad (which will then make the pad shape slightly skewed). Also, make sure the size of the ball pads follows the manufacturer specifications (including solder mask clearance)!