An alternative sensor:http://www.sparkfun.com/products/639
$25 for a module that does the "send a pulse/ wait for echo/ report time taken" stuff for you.
Evangalist warning: The people from whom Sparkfun buys this thinks it appropriate to put "Jesus First" on the PCB. If this bothers you, then you are warned, and can "vote with your wallet", but, please... if you feel that needs further discussion, take it to the thread already thriving at the Sparkfun product page?
Claimed to be able to report 20 distance readings per second.