We've been getting more and more questions about laying out the 0.4mm pitch Ti OMAP BGA, as is used in the Beagleboard . As I've written before, some of the rules change  at these tiny geometries. The  Beagleboard folks discovered that non soldermask defined pads (NSMD) can lead to bridging and poor yields and therefor they recommend soldermask defined (SMD) pads. Check out page 10 in their design guide . If in doubt, or if you're concerned that your set up might be different enough to warrant NSMD pads, I'd suggest you give a Ti Applications engineer a call.
And speaking of the Beagleboard, they just recently reduced the price on their pre-built Beagleboards. Like $125 for the original and $149 for the new xM version. Very nice.
If you've got a 0.4mm BGA part from a differnet manufacturer, check with that particular part manufacturer for the final say. Some 0.4mm pitch parts have a staggered arrangement  and in that case, there is enough room center to center to successfully use NSMD pads.
Joe Cool here.