Edge, HL2 was never, ever, initially intended for holiday release. They said sept 30 from day 1 at e3.
As for whether a lot of people will get HL2 through steam if it comes out substantially earlier...well, when Steam 1.0 came out, over 500,000 people downloaded it. So that's half a million people people who DO know what Steam is, and very likely would download HL2 if it meant playing a month earlier.