You can indeed do serious development using the Visual Studio 2008 express editions, this includes commercial products see question number 7 in the FAQ which says:
Seven) Can I use Express Editions for commercial use?
Yes, there are no
licensing restrictions for
applications built using Visual Studio
Express Editions.
The feature matrix shows that whilst you do lose some functionality between the Pro and Express Editions. The single biggest issue being that there is no add-in support (and adding it is forbidden by the EULA) which limits many nice additions to the environment such as ReSharper, VisualAssist, etc.
You also don't get a "Studio" but four individual editions, Web Developer, VB, VC++ and C#, if you wish to mix and match languages/projects in the way that the Standard/Professional Editions support then you are out of luck. Under the surface however, MSBuild is available and can provide you with multi-language solutions.