Update: March 22, 2015
[ Derek Molloy ]’s [ tutorial | around the 14th minute] (thanks to Andrew Dai’s reference) clarified my confusion on whether just powering up an I2C accelerometer would make its address come out. And that is correct, there are no special ingredients here. Just power up the chip and it should come out on the I2C bus.
For the GY-521 breakout of MPU-6050:
Powering up with either VDD_3V3 (P9.3 or .4) or SYS_5V (P9.7 or .8) works. Neither of these worked the last time I tried, so I wonder if I made a mistake in wiring the SCL and SDA (because the red LED was indeed coming on then, as it is on now.)
Note that the VDD_5V rail (P9.4 or .5) would not work if the board is powered up through USB, but the SYS_5V directly gets the USB power out to the header.
I am also being able to switch the MPU’s address. Yippee. Shorting ADO to ground (or even leaving it floating) configures the MPU’s address as 0x68, and pulling ADO to VCC (pull up resistor or just a wire) changes the address to 0x69.