I've heard that a colleague in Massachusetts has wondered what kind of ceremony is proper (or indeed, necessary) for a couple married in that church, but outside the benefit of law, once marriage licences are issued same-sex couples.
Golly: I wish I had that problem, but even though I don't, might I offer an opinion?
There needs to be a ceremony, however simple, so that the couple may contract marriage in the Commonwealth of Massachusetts. Until May 17, no same-sex couple, however devoted or ommitted, has had the opportunity to contract marage this way, and so in a real sense any prior service was different.
So what kind of service? The closest parallel we have is the blessing of a civil marriage, which is the same action in reverse. To review: the couple would have been married by law, and come to the church as married couple. The minister has the sole role of ecclesiastic agent: to bless, to lead prayers, and often times to lead the couple to exchange rings.
With the coming wave of previously-blessed same-sex marriages the minister serves as an agent of the court. (Indeed, from the couple's point of view, there's no particular reasons to return to church. The court clerk could officiate, but who are we kidding? You go to church to get married, right? Don't bother correcting me in the comments.)
This suggests the service should be as business-like: perhaps in the pastor's office, church parlor, or alternately, drawing from custom and if it was convenient, in the couple's home or the pastor's home
(really) with the following outline.