I just heard back from Greenfield, and this is there policy on daily surveys:

Daily surveys do indeed reset at midnight Eastern now. So you can take one at 11:55pm Tuesday and then take one 10 minutes later at 12:05am Wednesday and get credit for both.

The site should tell you when you have reached the maximum number of surveys per day (one). You can keep taking surveys until you get that message. You cannot, however, keep changing your email address to get more surveys. It would be considered fraud to use many different email addresses to be able to get multiple surveys.

So there it is. Greenfield continues to be great to work with, and we continue to see our survey numbers go up as we work with them to make the experience better on both their end and our end.