So, yes: caveats.
It is absolutely possible, as John states, to make money (even a lot of money) writing professionally. Also, note that I have never said you can't make money writing (even though it sort of looks like I did, and I should have been more clear). What I should have said was: you're probably not going to make a lot of money writing fiction.
John makes his money writing a variety of articles on everything from entertainment to personal finance, and building such a repertoire—in addition to the requisite (and now probably figurative) Rolodex of contacts and clients—takes a lot of time and effort. John mentions his starting salary as a journalist was very low, and sadly, things haven't changed much in the business since then.
At one time you could make a lot of money writing for television, and to some extent that's still true, but with the preponderance of unscripted "reality" TV shows out there, it looks like there are far fewer well-paying jobs in Hollywood than there were ten or twenty years ago. You could try screenplays, but it's a tough market made tougher by the recession and California's relentless budget crises.
I think your best bets are article writing (à la John), technical writing (writing manuals and... well, technical things), translation (are you fluent in a foreign language?), or copywriting/copyediting (requires a lot of experience and contacts to get good rates). Fiction, memoir, and poetry command lower rates (in descending order), and most people who stick exclusively to creative writing are lucky to make more than a few thousand dollars a year doing it. Also remember that, as a full-time writer, you are self-employed, meaning you're subject to self-employment tax and will have to pay for your own health insurance, retirement planning, &c.
Questions/praise/vitriol? To the comments!