While, I haven't yet seen it up close and personal, I've seen the announcement and datasheet for a new SoC (System on Chip), with Buetooth SMART BLE 4.1. The whole chip is 2.2 mm X 2.1 mm in a WLCSP (wafer level chip scale) packaged BGA. (Image from the Atmel web site). The part is the Atmel BTLC1000, with BLE and an ARM Cortex M0, running at 26 MHz.
It's got 32 balls, in a staggered pattern. That gives the chip a diagonal ball pitch of 0.3535 mm. That's really small. The staggered pattern isn't too common, but I expect that we'll be seeing more of it as parts continue to push the size envelope down.
Here at Screaming Circuits, We've built a few 0.3 mm pitch devices, and plenty of 0.4 mm pitch parts. I wouldn't anticipate any problems, but if you do get your mitts on one of the things, you should probably take the mitts off (because you'll loose the part) and call us and ask to talk it through with our manufacturing engineers before sending it in.
There was a demon in the oven. They said whoever solders him would fry. Their pads would bridge, their pc boards would twist and warp. The demon lived at 0.3 mm pitch on the metric scale, 0.0118121 inches SAE. But not at Screaming Circuits. That demon's afraid of us.