[theprgramerdude]

The ultrasonics really aren't that susceptible to interference - they're rather immune to it unless someone else has their robot directly pointing their sonar at yours. If that's the case, it'll probably only be for half a second max before the interference is completely gone and you can have good readings.

Edit: If you're really concerned about interference, then consider placing two, or even three, sensors to do the same job spaced across the robot. The multiple sensors will serve as an easy form of error detection in the event that interference is experienced.