Somehow, I don't buy the allergy/asthma excuse. In all his races I have seen him run, if he's not in contention to win or get a record he usually just flat out gives up.
Case in point:
Zurich (96?) 5000m: Komen and Geb are battling it out. 200-250m from the finish, when realizes he has no chance of winning, Geb almost comes to a stop...just jogging in.
Paris 03. Almost the same exact way, Geb gives up. Not even trying to finish with a hard effort (even if he is beaten).
It seems Geb is either at the very top of his game or just out of sight.
Another example:
Monaco 3000m in 94. After he broke the WR in the 5000m, he ran against Morceli. Morceli was handing it to him and then he just vanished from the screen running 7:37 and Morceli 7:25.
I think there is something to be said for just finishing the race with a respectable effort, not just run hard for 3/4 of the race and decide it's not worth finishing or keep continuing the effort.
The aura Geb had is definitely vanishing quickly. He needs to retire before he is way,way past his prime.