I haven't seen it either but I thought he got his memory back during the movie... I don't know. *shrug*
So I've been taking this far too seriously and I made a little... thing. It shows how at least some of the writings we've seen could have come from the same alphabet after all.

TADA!! Anyway, basically I've separated the symbols into groups at the top. Common means they're seen in at least two different writings and are therefore certainly all from the same alphabet.
The next group has the symbols seen only in the Five Doctors episode and identified as Old High Gallifreyan. However, some of the Old High symbols are also seen in other writings, so apparently not all of that particular alphabet has been forgotten. The ? group is symbols that I wasn't sure were Old High since I haven't seen that episode in a while and also the one at the end could just be a fancier looking version of the lightning bolt thing in the common group.
Next we have what was seen on the Rani's computer. Most of it looked rather weird and illegible to me, so I wasn't sure if I'd copied them correctly, hence the huge ? group.
Then, there's the things written by the Doctor himself. Not counting that long note from The Deadly Assassin... I'm not even attempting that.
xp Anyway, those are from his calling card and the "dig hole here" note. The ?s are because I wasn't sure if the first one was supposed to be a > and if the second one was one letter or two.
Finally, there's the letters from the first time the language is ever shown on the screen. These seem more like random scribbly things, but the one on the end does resemble the letter Omega without the little thingies on the bottom. Don't you love my technical terms?
razz Next, I spent an ungodly amount of time combing through lists of mathematical symbols (and a much shorter time looking at Greek letters) to identify them in Gallifreyan. I found that contrary to wha is said on the website I posted, the calling card does not have the letter Theta on it at all, as I demonstrated. There is a version of Theta which has the line touching the edges, but it's an even skinnier oval and the line is still horizontal.
Some odd things I noticed here (though more than likely unintentional by whoever wrote these things for the show) are as follows:
1. The weird curvy four found in (possibly) Old High Gallifreyan looks exactly like an upside down symbol for Planck's Constant which is something in quantum physics having to do with photons (too lazy to type out the whole explanation). Could the fact that it's upside down have any meaning? Also, the number four used early on could be said to be the same letter, only changed and simplified since the days of Rassilon.
2. The Doctor's calling card contains the symbols for the empty set (symbolic? look it up on wikipedia and you'll see what I mean) and the speed of light in a vacuum (which is supposedly the speed where time stops) followed by Sigma, which is apparently part of his academy nickname. Most of the other symbols seem to be made up or meaningless, though...
So, there you have it. I'm crazy.
lol