Answer: Dynamic .aspx sitemaps

According to the responses I received on the Google Webmaster Forums, it looks like Google doesn't care what the file extension is as long as... well here's the quote:

"The it doesn't matter what you use and what you call it - the content-type has to be application/xml or text/xml .

The server does what it does with that extension .aspx - it will execute the script. The script produces output in the xml format. You just need to ask your script to produce the correct header for content-type, before it starts outputting the xml content. So what the user agent gets is the output of the script. And what type it is - well, that's what content-type is for." - webado

Does that make sense to everyone? Good.

(thanks webado)