Windows Phone 7.8 Beta ROM for HTC HD7 Windows Phone device got released today. XDA-Developer forum member dotcompt has created this ROM with OS version 7.10.8835.35. The released ROm only on HD7, but Windows Phone Hacker has already been able to port it to the Samsung Focus.
Jaxbot from WindowsPhoneHacker has confirmed that this build is working and has the new start screen, but remains very unstable and is not really suitable for every day use.
On his investigation the only new features was the start screen and a new accent chooser (right), but given the mixed nature of the ROM this is not a reliable indicator of what Windows Phone 7.8 will consist off.
If you are ready to give this beta ROM a try, check out the source link for more details.