Craig Mazin has a [good article](http://artfulwriter.com/archives/2005/04/why_pitching_is.html) on [Artful Writer](http://artfulwriter.com) today about whether screenwriters are better off pitching their ideas, or just writing the script and trying to sell it as a spec. I largely agree with his points.
Keep in mind that Artful Writer is geared towards screenwriters who are already working in the industry, so the pitch-versus-write decision wouldn’t be the same for most aspiring screenwriters.