Let the hive mind of Engadget get that for you.
"As someone who doesn't reside in the USA, I was wondering what would be the best way to get internet for my computer in the US for a couple of weeks? If it were Europe I know I'd look for some prepaid data. Is there anything similar offered by American carriers? A MiFi or a data SIM that I can tether from would work, but I'm trying to maintain a tight budget. Help!"
The same thing happened to me a Circuit City when I was trying to purchase a Canon SD600 camera. I didn't get the $30 discount advertised on the website, so I asked about it, but was directed to "the only computer in the store with internet" which was using CC's INTRANET! The discount wasn't listed there, so I called home where my family verified that the discount was still on the external website.
CC employees told me I would have to go home to print the page showing the discount and bring it back just to get the discount. I finally found a laptop in the store that was left logged in to the store's network where I could access the internet. There I pulled up the page and showed an employee the discount.
After 45 mins of trouble, I finally got the discount, but CC DOES HAVE AN INTRANET AS WELL with different prices and discounts. Still, the employees were willing to let me prove the discount existed. Some seemed to know about the intranet and others didn't... so there's my story.