blog.msurveys.com

You can leave a digitial present (a text, photo, movie etc.) with this year's Christmas tree at the Tate Britain - visitors can interact with the tree in the gallery via their mobile phone (using Bluetooth), or send a text or file via email/web.
It doesn't say anything about being able to send an SMS to the tree, which seems a shame, considering how easy that would be to implement..

More info at the Tate Britain website (link above), instructions and a nice interface to upload a present via the web also available here.