Seems like wishful thinking. It's more likely that, if they were to release early at all, it would be on a platform most likely to make them money, such as one of the consoles. However, I think it's most likely that they'll release on all the platforms at the same time because of SteamPlay. Holding back Windows and Mac versions for a week for political reasons would just piss people off for a week.
The thing is, they make the most money on the platform they take the most money per sale from. That is always going to be Linux, because in the long run, Apple and Microsoft can lock down OSX / Windows and the consoles are already taking per sale cuts for MS / Sony. Valve can only bet on desktop Linux (aka, not Android) to never stop arbitrary application installation.
Well, no, they're not currently paying any licensing fees on Macintosh or Windows systems. So they make the same profit per sale for all three platforms. Now, this may not remain true forever - but if HL3 comes out before the status quo changes, pure profit wouldn't be the incentive (and in any case, in the current steam model you can buy stuff for Linux then run it on Windows later anyway). You could argue for driving people to Linux by giving them perks, of course - but that has nothing to do with present-day marginal costs.
Heck, make it a month. It's not like it's that uncommon to release a game with difference of months between launches for each platform. I'd see a lot of people buying Steamboxes if they made HL3, Portal 3 and the next L4D exclusive for them for a month or so.