I just got asked about this today.
If you will recall, in a previous post I explained why the CloseWorkbook() Excel Web Services method is a good candidate for calling in a "Fire and Forget" manner.
The explanation in that post was specific for VS2005. In VS2003, the name of the method that does an asynchronous call is "BeginCloseWorkbook" instead of "CloseWorkbookAsync". It is somewhat more complex to use (but only marginally) - if you ever did any sort of async programming in .NET, it should be second nature to you.