There are several threads on the Runners World trail running and ultra forums about training for races at altitude. You might want to search over there.

Try also: http://www.ultrunr.com/alt_ld.html
and
http://www.run100s.com/Freebies/Altitude.txt

Seems like the most important message is hydration. And I certainly do better above 10,000 if I avoid lots of protein the day before and day of and stick to simple foods (gel, maybe a low GI bar of some sort). I run in the 9000' to 11000' range quite a lot in the summer. 10,000 seems to be about the point where the run-walk intervals get shorter. I feel it in my legs more than anything. And then there's that whole getting enough air business.

I'm signed up for Imogene this year. Whether I actually run any of it or just hikerealfast to make the cutoff remains a mystery...