"The Fires of Pompeii" - Spoilery speculation on the Doctor's real name
My soothsaying of the longest-running secret in the Whoniverse, based on the soothsaying in this episode (clip posted below, for those of us in America who haven't seen the full episode yet):

"Your name burns in the stars ... in the Cascade of Medusa herself ..."

In "Last of the Time Lords," the Master mentioned that the Doctor had single-handedly sealed the rift at the Medusa Cascade.

In "The Shakespeare Code," we saw the literal power that words and numbers have in the NuWhoniverse, both to bind and to release certain forces.

In the podcast commentary on the BBC website for "The Girl in the Fireplace," writer Steven Moffat suggested that, since the Doctor doesn't even tell his closest companions his real name, there must be a "dreadful secret" about it.

And in The Making of Doctor Who, written by Terrance Dicks and Malcolm Hulke, and published in 1972, it was claimed that the Doctor's real name is a string of Greek and mathematical symbols.

Since I don't recall the Medusa Cascade ever being mentioned on either the classic or the current series before LotTL, it's entirely possible that the Doctor sealed the rift there very early on in his life.

So, here's my theory:

The Doctor's real name is the equation which keeps the forces contained at the Medusa Cascade, which he can never reveal again as a result, and the loss of that part of his identity was his first terrible sacrifice as a Time Lord.