In retrospect, the bigger problem when it comes to peddling your manuscript to reputable agents and publishers isn't likely going to be plagiarism (considering the number of submissions they receive, it's probably not worth the effort) but reaping the benefits of your copyrighted work.
Brian Keene for example has documented how Dorchester violated his copyright (and that of other authors):
http://www.briankeene.com/?p=6140
Kristine Kathryn Rusch also documents how some publishers have failed to properly compensate her for royalties (her website just suffered from a Wordpress attack/hack so I don't have a direct link right now).
One author who often litigates is Harlan Ellison, and while there are some battles he deserves to "win," there are some that surprises me as well.