The game has been spotted in the CDR database, but it's not yet officially available
After Valve launched the original Half-Life on Linux, now it seems that the original Counter-Strike will also make an appearance on Steam.Valve as probably just realized that most of their games have OpenGL support and the porting can be done by people in their lunch break.
After releasing Half-Life on Steam for Linux, Counter-Strike has made an appearance in the CDR database.
We can only hope that all the other Half-life related products, including all the add-ons, and even Half-Life 2, will be made available in the near future.
Even if a game shows up in the database, by no means is it completely certain that it's going to be available any time soon.
The CDR Database is a record of data that describes every Steam game and subscription provided by Valve for the Steam service. Usually, the new entries in the Steam database are first visible in the CDR Database before their official launch.