First time around, I hooked it all up and assumed it would work. Oh, the arrogance. The outgoing signals to the servo controller worked fine, but I couldn't seem to 'hear' the IMU from the Overo. I eventually bricked the IMU thanks to the unreliability of a $1 wall wart. I finally got the IMU back from the shop, and now I'm verifying the whole signal chain, front-to-back. For this purpose, there's nothing like an old-fashioned analog oscilloscope, and it so happens that I have a 60s-vintage Tek 454:
The first thing you'll note, aside from my ghostly, distorted face hovering over the trace, is that we can see that the IMU is indeed, transmitting. The 'scope is set for 2ms / sweep. Vertical scale: 5V/division. Therefore: period = 13 ms =~76 Hz @ +- 7.5V.
Horizontal: 2ms/div, Vertical: 5V/div.
So, we have a TTL Serial signal that looks to be about 8V idle, swinging to zero at the 'mark' bit (supply is 10V, btw). Scale it down to about 1.8V and we should be golden.