Yes, in about 2 days, either on Steam, or if that isn't ready by then, via torrents. Xen isn't finished yet 'cause they're completely redoing it from scratch rather than basing it on existing Half-Life levels, so you'll only be able to play up to Lambda Core.
Also:
(Yeah, I know Valve hasn't made BMS. :P)
Oh and speaking of Valve, apparently there's
a rumour going around the various news sites that EA tried to buy them in the past out for 1 billion dollars. And Gabe said that he'd rather let Valve disintegrate than be sold out by EA. What a guy.
Seriously though, that'd be the end of good PC gaming if that ever happened. EA just wants to make money, while Valve care about their customer base and listen to their fans. If Gabe ever resigns or dies or something, I hope somebody like-minded takes over from him.