I am having the same issue, apperantly Apple had an issue since this morning and I saw on a post that is back on however mine is still saying "waiting on Activation"......this SUCKS!!!
That is weird since I have friends that have the same phone and carrier and they haven't had any issues. Not that it is a big deal as regular messaging is working was just wondering if others were also experiencing the same problem. Hope they resolve it soon as I do like the blue messages much better.