According to London24 Emma Watson and Daniel Radcliffe have participated in a video that will be aired during the Olympics in London next month.  The article states:

A video featuring stars including actors Daniel Radcliffe and Emma Watson, musician Gary Barlow, boxing star Amir Khan and comedian and actor Simon Pegg will be played at the end of Olympic events. The five-minute video will be played on screens at a number of venues as spectators leave.

In the video Emma says: “I hope you had an awesome time, keep loving London.” They did not reveal Dan’s message.

(Other Potter stars may be involved as well, but the article only reports Dan and Emma)