Question: I'm running F17 and inside of `yum.repos.d`. I see multiple repos listed like `adobe-linux-1386.repo`, `fedora.repo`, `google-chrome.repo`, etc. When I `yum install` are some files being downloaded from multiple different repos or all from one?

Answer: 
Most of the repositories specify a `mirrorlist` in their configuration file. When present, `yum` will select one or more of the mirrors provided by the list. Repos that don't have mirrors will have `baseurl` instead of `mirrorlist`.

When downloading multiple packages, yum can download from multiple sites in parallel, though this isn't always obvious in the terminal unless you watch very carefully.
Also depends on whether a repo is disabled or enabled.