Here's a low-cost ultrasonic ranger with I2C and serial interface, using a single element, and a 55 degree sensing angle.
The Devantech SRF02 is compact and inexpensive single-element ultrasonic ranger featuring both I2C and serial interfaces. We're talking seriously inexpensive for a sensor with both these sensor interfaces.
Both interfaces allow you to chain up to 16 units together, and send your commands to specific modules in the chain. It's pretty straightforward to set the address of each unit too. Just plug in one module, and send it the appropriate re-address command with the new new address desired. Tada - finished!
A new feature is the addition of separate burst and ranging control commands, for even more control of the sensor's behavior.