Can't comment on the Nokia N900 , but i can talk about the iphone, Blackberry and Android platform, since between me, my wife and a coworker we have at least one or more of each. I started out using palms and windows mobile devices, but have given up on them over the years.
The iPhone is not popular because it has a cool factor. It is popular because it just works, works simply and most intuitively out of these 3 platforms. It also has the most wide target audience. Both my wife and I have a iPod Touch and like it for what it does and how it does it. We one Mac and several windows and a Linux machine at home, we only use the iPod Touch(es) with the Mac.
The Blackberry is great for business users. But my wife got it ( the 8900) for personal use because of its responsiveness ( over the myTouch Android platform).
When my wife got the Blackberry, I got the myTouch ( yes we are T-Mobile customers). In comparison to the iPhone and Blackberry platforms, the Android platform has been abysmal. I would have returned the device within the 2 week trial period, but the 1.6 update that downloaded the first nite I had the device, made the myTouch usable.
The Android reminds me a lot of using linux. One clicks and wonder if something is going to happen or not. Both the Blackberry and the iPhone give immediate feed back response.
Too often, going back to the home screen from an application leaves one staring at a black screen for too long ( 15-20 seconds).
In the 2-3 years of owning the ipod touch, accumulating 90+ apps, the device/apps glitched on me less than 5 times. In 5 days of using the Android, I have had more force quits than that.
I don't know if there are any UI guidelines for Android developers for touch screens, but some of the apps that come with the base platforms have poor widget choices for this form factor. Just setting the time for an appointment - incrementing/decrementing in individual minutes, and worse, when using the widget, the value one wants to see gets covered by your fingers.
I do like the Android's spell/word recognition system way better than what is on the ipod. It at least gives me words that I need to use, usually the most obvious choice. I don't know what the iPod uses to figure out what word to present, but it is not base on normal English usage ( and English is my second language).
I will keep the Android. I am a Google fan. I don't want a Blackberry, and I don't want to go to the extra expense of having the family on different carriers ( have 4 lines on a family plan). Could I afford the split I would get and use the iPhone.