It would be a very poor idea for Nokia to promise an upgrade to n900 owners now, to ease our minds, and not deliver later on rather than it would be for them to suprise us at, or close to, release with a shiny new piece of software for our phones.

That being said. Nokia does have a history of upgrading the tablets, and we know from the maemo summit that the hardware architechture, which is the commonly quoted reason for maemo 5 not being available on the older devices, will not be changing. As mentioned before, in the current climate where Apple and Google upgrade their devices, it would definately be to Nokia's benefit to follow suit lest they continue to see their market share decline in favor of more progressive software practices.

This applies to Symbian as well.