I believe somebody told you wrong.

There are some hotels that will upgrade you at checkin if they can, for example SRS Hotels says the following about United:

If you are a Premier®, Premier Executive® or 1K® member, you will also be offered a complimentary suite upgrade at check-in, if space is available.

but Hilton isn't one of them. Go to UAL.com, click on Mileage Plus partners, then hotels, and click on the ones to see what they offer.

[This message has been edited by cordelli (edited Nov 16, 2003).]