While Spotlight doesn't index Mail's Junk folder, if you use third-party anti-spam software that sorts spam into its own Spam folder -- such as Intego Personal Antispam, which is what I use -- your spam may get indexed. Once indexed, Spotlight will return some of your spam messages when searching for seemingly banal words, since so many spam messages have blocks of "normal" text in them. (Also, since I get about 500 spams per day, that's a lot of indexing time when mail arrives.)
So, to prevent Spotlight from indexing spam, open the Spotlight preferences, click the Privacy tab, then drag ~/Library -> Mail -> Mailboxes -> (name of your spam folder) to the list of items that Spotlight won't index.
[robg adds: There's a general hint in the system about using the Privacy tab to block some mailboxes. However, for those using third party anti-spam tools, this more specific version should cut down on the amount of cruft visible in your Spotlight search results.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060716052456952