The Woods Hole Research Center has been working on the â€œNational Biomass and Carbon Dataset for the year 2000 (NBCD2000) and have just released datasets from the first nine project mapping zones. All NBCD2000 data products can be downloaded on a zone-by-zone basis from the NBCD2000 project website. The datasets are free, but you do have to register at the site. The NBCD project is a really impressive undertaking, utilizing data from Landsat and SRTM, and derived data products like the National Land Cover Dataset 2001, LANDFIRE (vegetation), and the National Elevation Dataset to model biomass zones.
The team’s hope is to continue releasing datasets approximately one every week or so, until the project is completed sometime in 2009. There is an online mapping tool for viewing and querying the progress of the NBCD zone mapping (although it did not seem to be working correctly when I visited the site). A static map image of the current progress can be found here.