The SPT is promulgated, not negotiated, since, as was pointed out, there is no producer organization with which to negotiate. It is indeed in use - a great deal! According to the 07-08 annual report, SPT contracts accounted for over 29,000 work weeks, making it Equity's third most used agreement.
It runs from June through May every year. Equity again extended the current agreement this year. essentially freezing the minimum salaries (a few had to be slightly increased to meet some state minimum wage laws). My experience is that Equity welcomes suggestions from producers as to changes in the agreement, although they are under no obligation to agree to those suggestions.
The producer's category is based on the number of performances per week, and then by how many hours per week the producer wants to rehearse. For example, a theatre that programs four performances per week can use Category One, Two or Three, allowing them to rehearse up to 22, 26 or 30 hours per week respectively, with the minimum salary increasing accordingly.