Seeking Alpha's Sramana Mitra puts into words something like what I've had in the back of my mind for quite awhile now: it's possible that the iPhone could actually help WM. I'm not on board with her "the iPhone is actually laptop competition" argument, though. But consider this:
In a relatively small lakeside town in Minnesota, Excelsior, a new bar/restaurant opened up in December called Jake O'Connors. Everybody was thinking: this place is so nice that all the other bars and restaurants are going to have to shut down, there's just not enough space to go around. What actually happened, though, is that Jake O'Connors is doing great and all the nearby restaurants are doing better than they ever before. The new kid on the block made people want to come to that block, not necessarily just to see the new kid.
The basic idea here is that the smartphone market is growing -- quickly -- and it's likely that the iPhone will accelerate that growth. But the growth could come for the entire smartphone market, not just for the iPhone portion of it. There are a lot of feature phone users out there who are just starting to think "I need a smartphone", and the release of the iPhone could set them looking. I think its fair to say that there's a portion of those people who will choose WM over the iPhone.
It's like the old saw that opening a new fast food restaurant immediately next to a competing fast food restaurant actually improves business for both. A rising tide raises all ships. A bird in the hand.. wait, that last metaphor doesn't work.
So the iPhone may not spell doomsday for everybody else, after all. I agree with Mitra on one point - the iPhone's robust OS should make consumers demand a robust OS from their smartphones, whatever kind they decide on. Right now, Symbian, RIM, and the PalmOS don't cut it and that's an opening for Windows Mobile. There's another mixed metaphor for you - people tend to rise to the level of expectation; perhaps smartphones will too?