http://www.crsw.com/mark/Lists/Posts/Post.aspx?ID=32
Web.config modifications with a SharePoint feature
http://weblogs.asp.net/wesleybakker/archive/2009/01/21/web.config-modifications-with-a-sharepoint-feature.aspx
Using SPWebConfigModification to modify a SharePoint web application's web.config for all servers in the farm
add ajax httphandle to web.config
http://www.zimbio.com/VisualStudio/articles/5/Using+SPWebConfigModification+modify+SharePoint