It's not that BitCoin is unreliable, it's that it is still a currency in its infancy. It cannot be used to value things on its own, yet. No matter how much Bruce would like to have a set price in BTC that stays from the start of ticket sales to the end, it won't change the fact that there is still a good deal of volatility in the value of bitcoins. Volatility = a huge amount of risk when you are talking about playing with money you are borrowing (which is essentially what is happening when you accept payments up front for expenses in the future).
Risk can be kept at a low level while denominating price in BTC with some combination of selling bitcoins in advance, taking loans and buying options. These markets aren't very mature which is a problem, but Bruce could have found a creative solution if he wanted. I think denominating the price in USD is a strong negative signal and he should have spent the effort to avoid it. Even if it means making the price a bit higher - for many people the registration is about 1% of the cost of attendance so this pales in comparison to the signal.
Another problem is the bait-and-switch of listing the price in BTC, but then the order form is based on USD. Later he changed the listing to USD.
I again implore Bruce to update the agenda and the list of high-profile attendees.
PS. The reason I'm saying all this is that I really want to attend the "Bitcoin Conference & Worldwide Expo 2011 NYC", and went to some length to do so. But I don't want it to end up something else (a "meetup" or "LAN party" as some naysayers put it) due to poor organization and foresight, and this includes lack of attendance due to perception of said organization and foresight. Manny's comment that demand is strong is encouraging, but we need more.
That would have been practical if demand was a certainty. But with a first-time conference like this, it would be very difficult to know how many people would be attending, thus, you couldn't anticipate the right number of coins to borrow and sell.