They keep tweaking the website, from no longer accepting the first four digits of Russian Visa Infinite cards to later requiring a full sixteen digits of the card number in order to unlock the form (and validating that the numbers entered could be an actual card, though not verifying the account entered itself).
Each time they’ve tweaked the form, enterprising commenters have shared their experiences here.
Hello all. Just wanted to confirm that as of 9:52PM ET on 05/28/12 this is still working. Here’s what I did. First logged into my Hilton Honors account and changed my country to Australia and my Province/State to AP. I then went through the link to verify the card number. I tried dozens of card numbers but finally got this one to work -4423942965985959. I was then prompted to log into my HH Account. I logged out and back in and I went from Blue to Gold! Hope this helps someone else!