I'm a little late in responding to this, but here are some comments I made on a Treo oriented site:
Right now, I'm solidly in the WM camp. There are a lot of reasons why WM is more attractive than the other choices.
1) I won't buy an iPhone because of Steve Jobs'/Apple's attitude. It may be a great phone. But I rebel against anything that says "do it my way or you're doing it wrong".
2) Right now, anything Palm has three strikes against it, in my book:
a) Palm got rid of PalmOS, saying they were still going to strongly support WM in the Treo Pro & future devices. Almost immediately, they brought out WebOS/Pre & said they were no longer supporting WM. I have trust issues with any company that lies that easily or is that fickle (you can choose which).
b) the Pre & the Pixi don't impress me compared to other choices. They'd have to come out with something a lot better than presently existing WebOS phones.
c) Palm's decisions have made them a much weaker company, financially. Which makes one wonder if they'll be bright enough to come out with a much better WebOS phone & on a wider range of carriers. And, even if they do, based on previous choices, will they be around long enough to support anything?
3) Yes, the Nexus one & Android are really nice. But even with my experience with linux on the PC, not as easy to customize. And not as forgiving, from what I understand, if you decide you want to undo adding something to the phone's desktop. And, while Android is at version 2.1, most phones are still running 1.6 or earlier with no carrier efforts to update what's out there or hasn't been sold yet. Even though there's less of a hardware issue in doing so than WM hardware mixing with WM 7. That lack of updating is sad, since each update has added features & functionality that users miss out on.
4) While Symbian is mature, has lots of apps & custmizability, it still is bless flexible in those respects than WM.
5) WM 6.1 & 6.5 can do most of what each of the others can. There are apps to make WM look & act like any of the other OSes, without losing the features that haven't made it into the other OSes yet -- if you really want your phone to work differently than the stock version of WM. Except for iPhone's massive selection of apps (to some degree multiple duplicates of the same stuff), WM still has the best selection of mature, useful apps.
I'll now get off my soapbox.