Well, I don't have any credible sources. Someone once mentioned something like that to me, that's why I brought it up, but I have no facts. I'm just mentioning. That kinda thing is very common in the music industry, though, where a no-name submits a song to a label or whatever and they're "not interested", then a couple of years down the line, some big pop star comes out with a new hit single-- which was written by someone who was never acknowledged. Now that I have facts about, but books? meh. I just figured it can't be too far fetched...
Okay, I have heard of this. One agent blog mentioned the culprits were mainly other writers, so be selective about how much you post online about your novel. That agent's name I don't recall, but another guy's post I do know where I read it. Anyway, I'm going to find the agent who mentioned getting a copyright on anything you submit. I read it on Guide to Literary Agents Blog, so I'll be back with the link and the statement.
Edited to include the links
***here's the link for being cautious:
http://www.guidetoliteraryagents.com/blog/Is+It+Safe+To+EMail+Material.aspx
Another one by the same blogger:
http://www.guidetoliteraryagents.com/blog/default,month,2008-10.aspx
And here's the agent giving the copyright advice:
http://www.guidetoliteraryagents.com/blog/Agent+Advice+Paul+S+Levine+Of+Paul+S+Levine+Literary.aspx
It's just something to think about, I'm not saying a writer should or shouldn't do it, because after what happened with the woman who wrote a book to get kids to eat their veggies (I think it was called Deceptively Delicious or something close to that) and then Jerry Steinfeld's wife wrote one some months later that was eerily similar (even the cover reminded me of the other one), a copyright may not be enough.
***Edited to add
This case is probably is to what you originally stated. Seems the author sent the book proposal to the same publisher, was rejected and when she say the other book, filed a lawsuit
http://www.nytimes.com/2008/07/12/books/12sneak.html
"The “Sneaky Chef” dispute began last summer, when Ms. Lapine received an eight-page promotional brochure for “Deceptively Delicious,” a sort of mini-version of the book. Ms. Lapine said she was stunned to see the similarities between the books, down to Ms. Seinfeld’s cover (a winking chef and an attempt to hide carrots).
Ms. Lapine’s book had been rejected by HarperCollins and was eventually published in April 2007 by Running Press, an imprint of the Perseus Books Group. Six months later Ms. Seinfeld’s book was published. "