You have no idea how often that happens.
Chances are, you won't have to do anything since you spoke to the cashier and she let you off the hook.
You made a booboo, it happens.
Edit: Did some reading and apparently it is up to the gas station whether or not they pursue payment for the broken pump. It seems to me the girl was not trained in handling the situation because she should've alerted the manager and taken your contact information.
What brand of station did this occur at? This happens on average once a month at gas stations across the US and most stations leave it up to the managers whether or not they file a claim with insurance or press any charges. If the situation is handled poorly (like in this case where the cashier did not follow any type of procedure) it will be difficult for the station to put any blame on you. YOU talked to her, they'll have that on camera if they need to review it. She let you go without getting your contact info.
I wouldn't worry. If anything comes back to you make sure you've got your story and go into the situation with an apology. Those pumps are about $300 to fix and lord knows the gas companies make more than that every minute. The chances you'll have to pay for anything are minimal.
2011 SES 5spd #006 >> SR #213
What other people think of me is none of my business.