Sergei Gundorov (whom you may remember from a previous post) has published a paper on MSDN where he talks about a very interesting solution his team wrote in the Microsoft IT department. The solution is probably one of the most advanced implementation of Excel Services I have seen to date and includes lots of ingenious customizations - the white paper, specifically, talks about how to use Excel Services as a landing pad for data and then get the file stream from it, customize it (add stuff into it) and serve it up to the user.

You can find the paper on MSDN.