The fellow who developed it for the IESG is at Nokia and wrote a series of I-Ds last year on a URL for GSM Short Message Service.
Implementations are likely to differ between countries.
The original IPP notification spec had a provision for pager notification, and that was all we needed. I think that the special requirements of SMS may be similar to the Japanese i-mode. Perhaps Yuji Sasaki is interested in joining in. I think it is even more important than email notification. For them, an SMS notification is perfect.