I had an N810 (predecessor to the N900), and it was very cool - at the time, it really felt like "the future". However, I must admit that I liked it for what it was -a toy - and not for what it could actually do in a productive sense. At the end of the day, it wasn't possible to perform any sustained productive activity on it.
In broad strokes, for increasingly complex work, there is an inverse relationship between the portability and the productivity of a computing device.
This is particularly true for types work that involves mentally integrating and generating large (for the human mind) amounts of information like code - but also for reviewing documents, managing financial accounts (interacting with a large spreadsheet on a phone is a nightmare).
It's why at the end of the day, most engineers and architects (whether in software or real engineers in the physical realm), get most of their work done in front of large screens with full size keyboards (or at least large laptops).
This isn't to say that there aren't niche use cases or circumstances where a Linux computer in your pocket isn't extremely useful - the example always rolled out being "remote administration" of a server - but even these tend to be exceptions. It's nearly always better to accomplish remote administration tasks from a laptop.