Why on phones like the N70 can't you self-upgrade the phone's firmware like Windows mobiles can?
Why does the phone still have to be returned to a service centre for an upgrade?
Why can't you self-upgrade Nokia firmware?
Probably because Nokia isn't ready to support such a thing yet (whether it means tools, infrastructure, phone software or whatever).
It is easier to do for a smaller manufacturer that only has to support a handful models through a few operators with a pretty fixed set of hardware/features than one like Nokia that releases tens of models with hundreds of language/operator combinations every year.
I'm pretty sure we'll see Nokia doing something like this eventually (it has, for example, been written on http://www.s60.com/ that over-the-air, OTA, firmware updates are a built-in feature for S60 3rd Edition Feature Pack 1).