Durring the last betas right before version 1.0 was released, Jeb said on his twitter that he had raised the number of Strongholds from 3 per map to infinite. However, Mod authors who have reverse-engineered the 1.0 and 1.1 source code have concluded that he never actually made that change, and strongholds are still actually limited to 3 per map, and that they are still located in a ring around the origin, just like they were in the betas.
http://www.minecraftwiki.net/wiki/Stronghold
There is also speculation that "Ruins" of similar design to the strongholds are generated infinitely through the world, but I haven't heard of any confirmation from the source code about whether that is true.
Wozzek (Jabbercat) has already located one of the Strongholds. It is accessible deep under the basement of Oxford. It is fairly small, and contains no Library, but it does have Iron Bar windows, and silverfish.
Based on that location, we can estimate the location of the other two Strongholds. Neither of them is in a well-explored location, but they are both very close to well explored locations.
We can test this theory pretty quickly with a few careful tosses of Eyes of Ender. I have about 12 or 14 Eyes of Ender, but no Blaze rods. if anybody has a stash of Blaze Rods they want to share with me, let me know.
It is possible that when we find the other strongholds, they might have already been generated before 1.0, in which case our Portal To The End will be missing. If that happens, I will cheat with server commands and give us enough Ender Portal Blocks to build our own-- but I don't want to resort to that until we have found and explored all three Strongholds.