CBS has very few options available.
They can stonewall. However, it is now up to CBS to PROVE the authenticity of the documents. There are just too many questions about these documents to enable CBS to get by with the generic "we stand by our story".
They can fess up and admit that they were fooled by these fakes. They must then identify their "source". I doubt the docs lead back to Kerry, but it wouldn't surprise me if they can be traced back to Terry McAwful.
They can admit someone at CBS forged the documents.
If this turns out as badly as it looks now, CBS is in deep trouble. This is serious business. As a side effect, Rather has probably given Bush another few points in the election.
Oh, sweet irony.
I predict stonewall with a medium hang out.
CBS is in a lose lose situation. There best option is to neither admit guilt or push the issue. Simply come on one night and say there previous story with the documents have proven themselves to be suspicious and they until further notice retract it.