I just called Hilton to confirm my hotel reservation last night (make sure that they got it from Experient, add my HHonors account which wasn't an issue), and I found that the rate that Hilton was going to charge me was higher than what Experient quoted!
I somewhat paniced (since I'm staying 8 nights, it wasn't an insubstantial error at $20/night), and called Experient. Apparently, volunteers get $20/night off the "regular" FIRST rate for whatever hotel. This is the price quoted on the Experient website if you go in as a volunteer. The regular rate is $20 higher.
To put in concrete terms, I was quoted a rate of $133 for the Hilton at the Ballpark, and Hilton had it as $153. Make sure you don't pay more than you need to!
(and it's a nice gesture to give the volunteers a cut rate
)