I have visual Studio 2008.
I have noticed that I cam make WPF applications with Visual Studio. Does this mean that it can do the work that Expression Studio does?
I am going to install Visual Studio 2010. Does this do away with the necessity of Expression Studio?
Expression Studio (Specifically the Expression Blend portion) is geared toward the GUI creation. The interface in Blend allows for more rapid creation of the more sophisticated elements in WPF/XAML than Visual Studio does. And allows for easier creation of design time data, and databinding elements.
In Visual Studio, you can create the exact same interface, but much much more of it will have to be hand coded. What Visual Studio does do better is the code-behind and logic/CLR elements that support the program you are writing.
If you concern yourself with the GUI, I would say use Blend. If you concern yourself with the plumbing of the program, I would say use Visual Studio. If you do both, then use both. I use both when I am writing my stuff (i am a bit of a one man show) and it works out pretty well.