As you point out this is a developing area of the law, one of those relatively rare ones, IMHO, where it's appropriate for the courts to make some law and where the Fourteenth Amendment authorizes them to do so. The "he said, she said" situation can only be dealt with partially by cross-examination, which certainly can fail. Hence, the far better approach is to eliminate that problem at its source by requiring an advance written, signed and witnessed directive. The Schiavo case has done a wonderful job of illustrating the current infirmity of the law.
If it has done one thing positive, it is that.
If one spouse has doubts about another, or if it comes out (say at a family Thanksgiving or Christmas gathering) that there may be trouble ahead in this type of situation, a Living Will or Durable Power of Attorney is certainly in order. Especially if there's money at stake.
That said, we still need to address those situations where, for one reason or another, no such documentation exists. We can't just sit back and say, "Too bad. You should have thought of that before."
Without some legal procedure covering verbal requests, these cases will all turn into euthanasia cases where some (interested) third parties are going to be insisting on making the call.