A lot of functionality has been added in Web Tests in VS - 2008 Edition. Below is highlighted the major visible elements of this functionality.
VS 2008 : One major item is Auto Detection of Dynamic parameters and extracting them. This used to be a big problem in VS 2005, and this is not part of the inbuilt capability of the Product.
Another notable addition is the ability to create a Performance session for the test that you are currently working on, or one which is being actively being authored. See below.
BTW - I am heads down in work for next few months.