Today I found something usefull while surfing, a tool called BI Documenter.
http://www.bidocumenter.com/Public/Default.aspx
The current version can't be used for Reporting Services documentation, but the next can, according to the developers' blog. Great! I've wanted to find something like this for my RS reports for quite a while, so I'm looking forward to version 1.6.
I found it through Nick Barclay's blog:
http://nickbarclay.blogspot.com/2006/09/at-last-sql-2005-bi-documentation-tool.html