The FCC's investigation originally seemed to be focused on AT&T's role in rejecting apps. Had they been behind the decision it would clearly have been within the scope of FCC authority.
Having determined that it was solely Apple's decision to reject the Google Voice app, it's not even clear that the agency has any jurisdiction over the matter.
What may be more interesting is what, if any, affect the facts uncovered might have on the DMCA rulemaking process by the Library of Congress. The Electronic Frontier Foundation (EFF) is asking for an anti-circumvention exemption on jailbreaking mobile phones.
One of EFF's primary arguments is the anti-competitive nature of the App Store rejection process. The Google Voice decision would seem to underscore that point.
You can read the full text of the Google letter on the FCC website.