I will keep my N900 at least another year. After that I will buy whatever has similar features on more powerful hardware. IPhone or Android are no options for me. Symbian was my OS of choice before Maemo but I can't imagine going back to it now.

Must Haves for my next phone:
Linux/Unix based Open OS
Hardware Keyboard <- This is non negotiable
Browser like microB or better with up2date Flash
Integrated Skype and IM clients.
Ability to develop small tools on-device (Python or better C)
Stylus support
No bigger than 3,7" form factor (absolute max. = 4")
at least 32GB on device plus microSD slot

Nice to have would be
720p or better videocam
1GB Mobile DDR plus swap
Dual core
Draft N Wlan

Last but not least the build quality must be on par with Nokia or better. Metal casing would be nice. Flimsy trash like Samsung Galaxy S i9000 is out of the question.