At first thought this seems like a major blow for CodeWeavers and the commercial WINE consultation crowd; Steam and Valve games are among their staples, selling a large number of CrossOver licenses. However, on further consideration, if Valve wanted to make Steam/Linux a more seamless experience, one could see them becoming a major patron of CodeWeavers et al and/or becoming major WINE contributors themselves, in an attempt to ensure that most Steam games work perfectly on WINE.
This makes even more sense if one supposes that Valve ultimately intends to release a "Steambox" console. In such a condition, the question emerges: at what point does Microsoft feel WINE is damaging its profits sufficiently to revoke the free pass its enjoyed for a long while and bring out the legal banhammer? I think a Steambox that relied heavily on WINE would be excellent provocation.