By pure happenstance, I got a quick peek at a DSM-750 prototype yesterday. There was nothing confirming this in the About box, but it looked to me like the firmware is based on the old REDsonic code base used in the DSM-320 and DSM-520. The screen layouts essentially look the same with updated graphics that give it more of a Windows Media Center feel. Unfortunately, I didn't have access to a UPnP server with videos, so I wasn't able to determine if the dreaded Seek vs. Skip exclusive modes still exist or if they now support faster seek speeds and resuming playback.
Probably the biggest surprise was that it includes v2 of the Media Center Extender firmware (codenamed Pika), allowing it to run Media Center remotely from a Vista Home Premium or Vista Ultimate box just like the Xbox360. I'm personally not a big fan of MCX because it requires hosting a terminal server session on your PC for each connected device, which commits significantly more resources and processing power than serving a UPnP media session. In addition, the v1 MCX device I had wouldn't play Xvid/DivX videos, requiring you to transcode them to WMVs before being able to play them back.
Overall, I'd say I was pretty disappointed. The user interface on my DSM-510 is much nicer and with all the issues I've had with the DSM-520, I would have much preferred them using DigiOn's firmware for the DSM-750. I guess at this point I'll just have to hope that someday soon they'll update the DSM-510 with support for VC-1, H.264 and 802.11n.