While it's a separate team at Intel that works on the driver support for Poulsbo, Medfield, and other chips with the graphics core being derived from Imagination PowerVR SGX technology, Eugeni Dodonov pointed out that the PowerVR VA-API drop from this past September should work with Intel Medfield / Penwell.
With Intel seeking your feedback and questions (he's still accepting questions from Phoronix readers), it's been mostly about the in-house Intel graphics solutions like Ironlake, Sandy Bridge, and Ivy Bridge. Eugeni mentioned today though that the PowerVR drop for Q3'2011 "should support Penwell and Medfield platforms."
The code in question is the vaapi/pvr-driver Git repository. The repository only has a single commit and it's titled "pvr: initial video driver drop for IMG VXD/VXE video engines."
This PowerVR VA-API driver is advertised as offering hardware decode acceleration for MPEG4/2/H264/VC1 on the IMG VXD375/385 hardware. There's also encode hardware acceleration for MPEG4/H264/H263 with the IMG VXE250/285. The video post-processing is handled by DC/overlay and GPU. The VA-API driver is reported to work with MeeGo and Google Android. The author of the driver itself was Austin Yuan at Intel.
The PowerVR VA-API driver is open-source, but this is just the user-space bits for video support. Intel's Alan Cox with his open-source Poulsbo driver has said that implementing open-source video acceleration support might be possible with the information that is publicly available and not covered by the hand of Imagination Technologies.
At this point it looks like the rest of the Medfield Linux driver stack is a binary mess, but it's hard to tell as Intel isn't interested in sharing the whole scoop and the hardware was just publicly unveiled last week at the Consumer Electronics Show with limited availability thus far.