For me the Nokia n900 alternative is the HTC Hd2 which is an absolutely wonderful phone. Also the screen is amazing. HTC Hd2 offers the unlimited hack ability and possiblities that Nokia n900 offers Xda Developers has done wonders for the HTC Hd2.
Well the HTC Hd2 has practically the same camera 5mp with dual led flash
Opera is included with the phone and works pretty well also you can download Skyfire for viewing flash content.
Video playback is stunning on the HTC hd2's 4.3 inch screen.
The HTC Hd2 does that very well.
What phone can't do email. Email should pretty well on it being as it does have a very large screen so it should be very easy to read.
You can't do video calls on HTC hd2 but you can download Skype or Fring for the HTC h2 and do voip just fine.
The HTC Hd2 will be very usable in a year.
HTC Hd2 does include wise pilot navigation that works pretty well. From what phones I have seen the HTC Hd2 is the best Nokia n900 alternative. The best thing is that HTC Hd2 has freaking portrait mode and mms I don't understand why it is so hard for Nokia to deliver that. I consider both the HTC hd2 and the Nokia n900 "mobile computers" or "pocktpcs".