I have a password protected site. Can the webcrawlers - for instance
those that do the indexing for google - get onto the site and index
what is 'behind' the password protected area? Specifically, can they
index materials that are put there for downloading?

If a human cannot access the files without a password, then a web
crawler cannot either.

That is not necessarily the case, but more often than not it is.  A
lot depends on the security mechanism implemented by the developer.

To make sure that your pages are not followed by the google crawler,
I'd be sure to include a robots.txt files, along with appropriate
metatags in your source for the password protected areas.

To read more about robots.txt, and the metatags, check out the following links:

description of robots.txt
====================================================
http://www.robotstxt.org/wc/exclusion.html#robotstxt

more on robots.txt
====================================================
http://www.searchengineworld.com/robots/robots_tutorial.htm

description of relevant metatags
====================================================
http://www.robotstxt.org/wc/exclusion.html#meta

standards for robots/crawlers exclustion
====================================================
http://www.robotstxt.org/wc/norobots.html

Thanks all! I think that does it.