I haven't read Eragon, and I don't intend to. The history of its publication is similar to Daemon and Scratch Beginnings, the two self-published books I read recently, and if its quality is also similar, I have better things to do with my time. Besides, I'm more of a science fiction than fantasy man. And I spell "dragon" with a "D."
Anyway. Teenage author Christopher Paolini's parents printed their son's first novel and marketed it themselves, but by all accounts Eragon was not considered a success (or, I imagine, remotely profitable) until Knopf acquired it and reissued it in hardcover. I'll admit I haven't done extensive research, but I've yet to hear of a single self-published author who turned down an agent or editor after attracting media attention.
It seems pretty clear that "self-publishing" is a misnomer; it's really just printing up bound versions of your manuscript, which may or may not be any good, and then selling it yourself. It's no different from the people who make arts and crafts to sell at swap meets or street fairs or on Etsy, except that there is some status associated with being "an author" and not just "a writer."
There's nothing wrong with printing your own book--we've done it with our 2008 road trip photos and the Hogwarts Game textbook, and every year I print a copy of my finished NaNoWriMo novel because my wife doesn't like reading 50,000+ words in Courier font. But that's not publishing. That's printing. We're doing these things for fun, not as a business.
If you're crafty and like making things, it can be a lot of fun to make a book and sell it at your local flea market. Print on demand (POD) services are great for low-volume, special-interest items like the Hogwarts textbook. Ironically, we moved 107 copies of that thing in 2006, which makes it more successful than many actual, published books:
"Here's the reality of the book industry: in 2004, 950,000 titles out of the 1.2 million [in print] tracked by Nielsen Bookscan sold fewer than 99 copies. Another 200,000 sold fewer than 1,000 copies. Only 25,000 sold more than 5,000 copies. The average book in America sells about 500 copies."
-- Publishers Weekly, July 17, 2006
Of course, we weren't trying to turn a profit, or even offer the book as a separate product--it's just a souvenir of The Game. We also want to stay under the radar so Ms. Rowling's lawyers don't come after us.