1. dont know
2. N900 supports active sync which google does too.
3. adblock plus is already available.
4. dont know
5. there is no native app for the google voice but can access through web.
6. active sync is available.
7. Video of tether to Ubuntu and Mac is available in youtube. not sure on PC side yet.
8. dont know
9. just swap the sim card and call customer care (611) tell them you want to switch to smartphone plan and you are done.
10. Lot of other threads in the forum which discusses this. search