There's a new article up on MSDN that describes a custom solution built by the IT department here at Microsoft that uses Excel Services. Rather than re-paraphrase, I'll "borrow" liberally from Shahar's description:

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.

The article can be found here.