If you guys follow my post i discussed about marten's way of converting clasic worksheet to extended worksheet in http://blogs.msdn.com/pranavwagh/archive/2006/02/05/525177.aspx and then had a discussion with andrew whitechappel that is this feature comming in next release. i also blogged about it in http://blogs.msdn.com/pranavwagh/archive/2006/02/07/526084.aspx after i came to know that VSTO v3 is released, asked andrew once more about the update and in his own words
"We're still looking at the possibility of creating VSTO codebehind worksheets programmatically - no decision yet. We'd like to do this, but there are so very many things we'd like to do in this release, and we can't do all of them."
have a look in the comments section in http://blogs.msdn.com/andreww/archive/2006/03/25/561013.aspx