Speech quality using the "HD Voice" (aka AMR-WB / G.722.2 codec) is much improved - listen for yourself in this BBC News story (possibly UK only).

The question is: does the N900 support the AMR-WB codec?

My web searches so far seem to suggest that the N900 does not support this voice codec, which is a great shame particularly as AMR-WB has been around at least 10 years (possibly longer, since 1998).

The latest Android phones (Moto Droid etc.) and forthcoming Nokia N8 seem to have support for AMR-WB, but not Nokias current flagship.