Another reading of the wiring instructions (GitHub - pololu/lsm303-arduino: Arduino library for Pololu LSM303 boards) shows my mistake: I plugged SCL & SDA into digital 4 & 5 instead of analog 4 & 5. Embarrassing.
Suggested wiring:
Arduino Uno/Duemilanove LSM303 Carrier
5V -> VIN
GND -> GND
Analog Pin 5 -> SCL
Analog Pin 4 -> SDA
My original wiring:
Now that it's wired correctly, it works fine.
I will keep in mind your suggestions when troubleshooting future I2C problems, Nick--thanks.