this is very naive. if it is as simple to add a keyboard to mobile device as expressed above, there would be much more qwerty - keyboards in them....

1) physical qwerty vs no, major difference in hardware i would say
2) microsd - slot
3) removable battery (increases device size atleast)
4) keyboard is very light, but a mechanism for the keyboard is another thing if you want it to feel solid, not like £2 chinese slide form phone.
5) what was iphones resolution again? n900 has 800x480
6) connectors (tv-out, standard ones vs iphones propietary,....)
and so on...

so what is common @ hardware?
-processor.
-amount of memory (e:ram) if i remember correct
-storage size when talking about iphone 3gs 32gb