The reason Intel developed the FSB-multiplier lock was to prevent unscrupulous OEMs and computer shops from selling overclocked CPUs falsely identified as higher-performing parts.
Intel fully expects the X7800 and X7900 to be overclocked. There is no reason to pay the price premium for them, otherwise, because the stock performance isn't worth it. However, by not warrantying them beyond their "rated" spec, Intel shifts the support costs to the OEMs and end-users.
Intel could just improve their binning a bit and find the X7800s/X7900s that can run reliably faster then 2.6/2.8GHz, but then they might not be able to meet demand for these "faster" chips depending on each wafer's yield. But as the process improves, speeds naturally rise, so now that the majority of Core2 Extreme Mobiles coming off the wafer reliably run at (least at) 2.8GHz, Intel has now created the X7900. When the majority reliably run at 3.0GHz, we will see the "X8000" or somesuch. Then the "X8200" when they hit 3.2GHz.
But before they officially launch from Intel, folks will be running X7800s at 2.8GHz (or beyond), X7900s at 3.0GHz, X8000s at 3.2GHz, and so on. How far "beyond" will depend on cooling and other factors.