I'm into my second year of my phone contract and plan to keep my n900 well beyond that.

Currently I use my n900:
Logitect PS3 Media Pro Keyboard/trackpad with ...
...Easy Debian - "Linux in my pocket"
Emulators: GBA, VB, C64, ZX Spectrum
SunVox - amazing app to make music
other ported Linux software too.
I've also overclocked my n900 to 900mhz (makes a massive difference)

At the moment no other phone offers me any of this in a meaningful way.

Meego is currently in development and I'll quite happily wait and see how is eventually develops.

If I really had to choose a different phone it would probably be android-based.
Unfortunately it is almost impossible to port Linux-based software over to it. That's the only thing stopping me from switching.