IANAL and I have not followed any relevant case law BUT...
It would seem to me that "software licenses" (in lieu of sales) are a dubious notion. Why? No signed contract and thus no formal registration (no way for the company to prove who owns which license). No way to prove the signer is over the age of 18 years. No way for the consumer to get a refund if you refuse the terms of the license (since most stores refuse refunds and only allow exchanges) but the license cannot be read or agreed to prior to purchase.
Now, since most software, ebooks, mp3, etc are sold as "licenses" and not sales, if this concept is ever dismissed in court, then by default the "First Sale Doctrine" would apply to all digital files (media, software, etc.) since they are undoubtedly selling you something, i.e. requiring money before the user is granted access to the product. And thus DRM would be illegal because it violates part of the consumer's First Sale Doctrine rights.
Of course, this would create bloody hell in the courts...not just over digital media, but things like cars (with proprietary software that the user cannot access, only dealers) and a number of other business process patents.
But it sure could be interesting.