Aside from the whole ABC/ESPN thing, I think it's a bit much to assume that they are trying to cover up the demise of the Big 12. The ACC raided the Big East and there was no cover up. It's business. Why would anyone care?
Also even if that was true and did happen, the negative effect to the SEC is greatly exxagerrated. SEC football is the strongest brand right now because it has the best football. If the Pac-10 expands, then the SEC maybe slips to #2 but still well ahead of the Big 10. The TV demand will still be there. The average guy is going to want to watch Bama vs. Florida over OSU vs. Nebraska or UT vs. UGA instead of Iowa vs. Missouri.
Finally, anyone that thinks UNC and Duke are leaving the ACC or that the SEC is inviting them is smoking crack. Their football programs are nowhere near SEC level. Have you seen Wallace-Wade stadium? DB and Science Hill have better facilities. Duke gave away 1,000s of free tickets to a game last year and still no one came. Plus they have to many other BBall rivalries in the ACC that they probably don't want to abandon.