Great post about new features of ASP.NET 4.0 and VS 2010 Beta 1 can be found here:
Read more about:
Shrinking Session State
The ability to set meta tags
Support for using ASP.NET routing with Web Forms.
More control over generated IDs.
The ability to persist selected rows in data controls.
More control over rendered HTML in the FormView and ListView controls.
Filtering support for data source controls.
Web configuration-file transformation