The "not being left behind" and "it's open source, so we could always continue working on it if Nokia stops" argument doesn't hold water. Look at all the threads about N770, N800, N810, and N900 being left behind as Nokia rolls out the next OS version. These releases, for the most part, cannot be backported into any kind of official release. There's been hacker editions (which is great, to Nokia's credit), but at this point, the N770, N800, and N810 are definitely left behind. Whether the N900 will be left behind in the Maemo to Meego transition has been in question since Meego's announcement. The OS is open, but not open enough for others to continue.

For developers, Nokia has been good with keeping them up-to-date with discounted hardware. For end users, I would venture to say that official support is for about 1.5 years after product release. If this is your typical upgrade cycle, then the N series is perfect for you; otherwise, buy N series products if it has all the features you want, not features you hope to one day have. It's about immediate utility versus possibilities.

The iPhone 4G, if the specs are true, finally has specs comparable to the N900, and maybe that's what the OP is having a quagmire about.