The fact that they haven't suggests to me that they'll be killer. Why? Because every bit of news thus far has been better than the last—better batteries and screen; a YouTube client; Word, Excel and PDF support. They're masterfully playing the media. I've been by the Apple Store a couple times in the last few days (my sister just got a MacBook), and there's always someone standing and staring at the giant iPhone displays.
If it was going to be bad news, Apple could've buried it in a press release three months ago. But instead they're holding it until launch week—some people think it won't even be until launch day. Bad news then would shrink their lines quite a bit, so why would they hold it back?
It also helps that AT&T isn't subsidizing the phone; that means they can get by with a lower service charge.
My guess is that we'll see packaging very soon (Apple always shows their packaging, look at any of the iPod launches) and a rather remarkable monthly rate no sooner than Thursday.
(I won't be in line Friday, though—at least, not unless a series of very unlikely events occurs, including both a better plan than I think they could possibly afford and a firm decision on college.)
EDIT: They released it today. It's fairly good, but nowhere near the sort of amazing I'd need to be able to afford it. Le sigh.