CC-BY has nothing to do with DRM. It should be easy to use CC-BY assets in a commercial game.

(edit)

Correction, there is a clause in CC-BY as well:

"You may not impose any effective technological measures on the Work that restrict the ability of a recipient of the Work from You to exercise the rights granted to that recipient under the terms of the License."

This clause makes a lot of sense for CC-BY-SA, but I'm not sure what it actually means for CC-BY.

I imagine a safe way to handle this is to include an extra non-DRM copy of all the CC-BY assets used with the distributable.

When You Distribute or Publicly Perform the Work, You may not impose any effective technological measures on the Work that restrict the ability of a recipient of the Work from You to exercise the rights granted to that recipient under the terms of the License.

by-sa/3.0/legalcode (4(a))

I too consider this to have to do with DRM but I'm wondering if it's not enough if you add info on where to get the relevant in-game data DRM-free in the credits or something.

I think it's possible to not use DRM on Steam.

I think CC-By does indeed have some sorts of limits on DRM.  But I don't know how extensive they are, or whether there are easy ways around them, which there may be.  Generally, if you're having trouble with that you can just contact the artist who can give you individual permissions to override issues on that front.

just contact the artist

To me, not having to contact the artist is a major benefit of freely licensed art: permissions are already clearly (EDIT: OK, maybe not so clear...) laid out.

True.  But let's be honest: even our best licenses kinda suck sometimes ATM.  Just look at all the confusion over what -By-SA means for licensing of other game assets.

Funny thing, I was just talking about how DRM is one of the reasons I stopped buying games. The last games I bought were DRM free humble indie bundle games that had free(as in speech) engines.

http://commons.wikimedia.org/wiki/Commons:Licensing