Unfortunately there is not a way to embed with HD on by default at this time. Many people complain about the playback stuttering as it buffers which is why we don't default to the highest quality anymore.
Thanks for the speedy response! Do you know if you have any future plans to have this available as an embed option? I imagine it would be quite simple to implement as your reply suggests that it defaulted to HD on in the past.
The reason I've opened an account is because the Vimeo standard compression quality has dropped recently so I am looking to migrate my content. Having the HD embed option may entice people to Creative Cow who are looking for an Vimeo alternative.
Yes, we are considering this when we update the videos section with new embed controls. I've added an Easter egg for you to access this functionality. If you add ?hd to the end of the embed URL it will turn hd on by default.