You should read the EULA and consult a lawyer on this.
As far as I know, Valve's policy in the past has been...
Any content (sounds, sprites, models, textures, etc) from a Valve owned MOD are freely allowed to be used by MOD authors as long as the MOD will run on the Half-Life engine. (i.e. you can't use Valve's copyrighted content for some non-Half-Life game).
Has Valve's position on this issue changed with Steam and/or Condition Zero? Some Condition Zero content is probably owned by Ritual (unless Valve's contract with Ritual stated that Valve owns any content developed for that game).
I would post a link to the e-mail message that Ken and Eric sent when this question was raised on the old www.topica.com hlcoder list (where they stated that using Valve content was okay for MODs running on the Half-Life engine), but www.topica.com no longer has the "hlcoders" list archived on their website.
-- Jeffrey "botman" Broome
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