In general, we and just about everyone else on the planet recommend NSMD (non solder mask defined) pads for your BGA land. Using copper to define the land gives more precision that using solder mask and, more importantly, gives a more reliable solder joint.
There are a few cases where SMD (solder mask defined) pads might be more appropriate. The Beagleboard folks, with their 0.4mm pitch Ti OMAP processor found that at that small a pitch, NSMD pads increased the chances of bridging.
Interestingly, IPC writes that the main advantage of SMD pads is to prevent pad lifting. They further note that since the center balls are most likely to have lift issues, due to the greater concentration of stresses. If you have concerns about corner pad lifting, you might want to use SMD pads just in the corner for a little extra holding power. (ref IPC-7095B, 6.2.2)
Danger! 50,000 Ohms