This is a quote from the Media SDK 2.0 marketing materials: The library API is cross-platform and cross-operating systems with software fallback.
Please, specify the content of the terms: "cross-platform" and "cross-operating systems". Cross-platform is sandy/ivy with hd 3000/4000 and Cross-OS is Vista/W7/W8? Correct?
Unrelated to Peter's authoritative answer, Intel has released a solution for hardware decoding AND encoding under Linux, called LibVA (or VAAPI). I have little experience under Linux, but the documentation seem to indicate the functionality is comparable to MediaSDK, even if the API is quite different.
libVA indeed provides low level access to some codec HW acceleration. But unfortunately, the current available Linux graphics drivers does not provide a very useful level of performance or quality. I do not expect this to improve.