We are curious in current practice in SMS use - and I hope for a good reason. Together with Jonna Hakkila and her group at Nokia Research we have discussed ideas how to make SMS a bit more emotional. Hopefully we have soon a public beta of a small program out.
Till then it would be helpful to understand better how people use SMS and how the encode emotion in a very rudimentary way by :-) and :-( and alike. If you are curious, too and if you have 10 minutes it would be great to complete our survey: http://www.pcuie.uni-due.de/uieub/index.php?sid=74887#
Emotions in SMS and mobile communication has been a topic many people have been looking in, one of the early paper (in fact a design sketch not really a paper) was by Fagerberg et al.  in our 2004 special issue on tangible UIs - an extended and more conceptual discussion of their work can be found in ; for more on their project see: http://www.sics.se/~petra/eMoto/
 Petra Fagerberg, Anna Ståhl, and Kristina Höök (2004) eMoto - Emotionally Engaging Interaction, Design Sketch in Journal of Personal and Ubiquitous Computing, Special Issue on Tangible Interfaces in Perspective, Springer.
 Ståhl, A., Sundström, P., and Höök, K. 2005. A foundation for emotional expressivity. In Proceedings of the 2005 Conference on Designing For User Experience (San Francisco, California, November 03 - 05, 2005). Designing For User Experiences, vol. 135. AIGA: American Institute of Graphic Arts, New York, NY, 33.