I've gone to High Hrothgar looking for him to get directions to another word of power. I've done it before. I fear he's taken a spill over the edge of the cliff. There are only two greybeards in High Hrothgar at this time: Borri and Einarth. No one else. I've gone back at different times. Same two and only those two. I'm on the 360.

Did this just mess up the Main Quest?

I've seen strange stuff happen before. Like when I killed Audens Avidius in Oblivion AFTER turning in the evidence and getting the witnesses all lined up. They arrested his corpse. I went to his trial and his corpse was there. I visited his corpse in prison. And I saw his corpse at the front door to the prison after it "escaped". Hilarious.

I don't want to do a search around the High Hrothgar to see if he did fall. That would be bad. Very bad. It's a long way down. I may do the "Life" shout and see how many show up. Maybe he's stuck in a wall. That's just as bad. On a 360 -- you know, Beth, you should have some sort of console command available for the game consoles.

He annoyingly moves around a lot. Look for him in the side halls kneeling down, or in the middle, or behind the statue that's in the middle area. Use the "wait" function and skip by 1 hour every time to see if that works. It's what I did when I didn't feel like chasing him down.

Oh, and he can also be sleeping.

Did you check the rooms and the courtyard? Surely he can't 'disappear'?

It's so dark in there he could be laying in a bed and be hard to notice.

He sometimes goes up the tower in the courtyard.

Up the tower in the courtyard. Hmmm. didn't look there. I did spend about a half hour looking for him. I looked in the side corridors, in the beds, under the tables, in the book cases, outside. I'll check there next time.

i just wait until about 1am and he's generally in bed.

OMG.... I WISH I'd had that happen, I would have laughed so hard I'd have fallen out of my chair! (Not to mention the hysteria engendered by having to spend an hour trying to explain my hilarity to non-game-playing non-computer-using husband....)