Sprig - since this is an advanced release of Mistborn, we are doing reservations. It has several purposes. Mainly, we want to order from Tor only the copies needed. By having a reserve list, be can be more accurate in that ordering process.
If we order too few, fans get mad at us, and more unfortunatly get mad at Brandon sometimes.
If we order too many, then we have a huge backstock to sell, and have to dig ourselves out of a hole.
It also gives us an idea of how many people to expect that evening. There are other people coming that night to help promote the event, and they need as much info. as possible.
From a marketing standpoint, a reservation list builds awareness and excitement. The more excited people are, the more likely they are to tell their friends about it - thus generating even more awareness and excitement.
Apart from that, Brandon is a local favorite. Just so people understand, Elantris outsold the new books from guys like Martin, Jordan, Hobb, Goodkind (easily), Pratchett, Scott Card, etc. I am glad he has been nominated for the Campbell, because Elantris could have sold even more copies if people in bookstores did their jobs. Barnes and Noble names Elantris the best new SF/Fantasy of the year, and they have maybe 2 copies in their stores. Borders was excited when they sold 5 of his books at a nook signing (that was sucessful to them). Elantris could have sold double what it did. So, Mistborn - with more press, and more readers - should sell a TON more. I want to make sure it does at my store.
That's why there is a reserve list, and that's why he deserves the attention.