At what date (and/or event) was the Old Testament canon currently recognized by most Protestants finalized? What is the earliest occurrence we have of it being referenced in it's current form and who (or what event) announced it as finalized?
According to Talmud, the Tanakh canon (Protestant OT) compiled in 450 BC, but modern scholar puts the canon as a process between 200 BC to 200 AD. First the Torah, then Nevi'im and the latest is the Ketuvim.
For a short answer to the question,
You can see the statement in http://www.reformed.org/documents/wcf_with_proofs.
For the long explanation, you can look at this wiki article:
In Jesus' time there was no one universal canon for all Jews, but two competing ones, which I list below.
The Masoretic Text (http://en.wikipedia.org/wiki/Masoretic_Text) formed the basis for the Protestant Bible, for example, and is what the Jews use as authoritative.
The Septuagint (http://en.wikipedia.org/wiki/Septuagint) includes the deuterocanonical books, and was quoted by the Apostles and the Early Church Fathers, but that does not imply they they took it as being inspired, but they were familiar with them.
During Luther's time there were disagreements as to what to include in the OT, and this continued until 1647 when it appears the final change to the OT canon for the Protestant church was put to rest.