Actually, the fact that it's a Source game on Steam pretty much confirms that there will be a multiplayer crack not long after release.
Valve tend to be fairly open about the inner-workings of their games. Any Steam game can be made nonsteam by simply extracting some files. If it uses the current Source server system it'll be supported by Setti's alternative masterserver, which only requires the user to overwrite a text file. If it doesn't, there's plenty of other ways, such as undead patch and lan exploits.
Valve tend to be fairly open about the inner-workings of their games. Any Steam game can be made nonsteam by simply extracting some files. If it uses the current Source server system it'll be supported by Setti's alternative masterserver, which only requires the user to overwrite a text file. If it doesn't, there's plenty of other ways, such as undead patch and lan exploits.