Have been triggered to thinking about this for a number of reasons. Partly it was the Dron book about so-called linear media. Txt messages are not free but peekamo makes it as near-to as could be imagined.
http://dx.doi.org/10.1016/j.compedu.2005.11.014 have some good hints about unexpected outcomes. It seems that the time and cost of making an sms are factors of concern to students with many using pay as you go phones. It would make sense to give students 2 minutes at various points in the lecture to txt questions/comments.
One other issue raised is the fuzzing of boundaries between life outside the lecture hall and life inside it where students have subjected themselves to the pedagogic control of the lecturer and may be discomfited by him/her opening the lecture up in this way: normally we encourage students to turn their phones off! I liked the way that the messages were then made available after the in-class session in a discussion-type interface to encourage even more interaction.
I wonder if it might be possible to bring elements of the DFAQ system (see http://www.ifets.info/journals/7_3/5.pdf ) together with this...