Psirens is converted to XNA 4.0, and it was about a half day to get it running.
It looks like crap (culling, shading and transparancy problems). This will take some time to fix….
Here is a collection of links to help with the XNA 3.1 to 4.0 conversion:
http://www.hexworld.co.uk/blog/?p=111
http://blogs.msdn.com/b/shawnhar/archive/2010/04/19/vertex-data-in-xna-game-studio-4-0.aspx
The blog of shawn hargreaves explains a lot so check his entire site for info.