Two things,
Interesting that if Wanjiru really did have that strategy, Makau did something similar in Berlin where he said he slowed down hoping that the people ahead of him would speed up and burn themselves out thinking now it was the time to break away. However as he said it didn't work so he went back to the lead pack. Wanjiru seemed to do the same thing by allowing Kebede to burn himself out and then finally overtaking him.
Secondly, for once there really is an excuse for a slower time. Wanjiru knew he wasn't in shape to run fast, that's why he picked Chicago over Berlin (which he probably would have lost). I'm not surprised that he waited to the end, wanting to spend as little energy as possible just to get the win (which is again why he switched races) so he can be all the more prepared for the WR assault in Dubai. Yes it was completely for a pay day so he could run slower, but it was part of his plans and hopefully the win (which seemed like it was easy since it came down to a kicker's race) didn't take too much out of him so he can be ready for a sick race in January.
Btw his marathon preparation is impeccable. It's like he's a complete marathon specialist. I think ideally one would be in their best shape for just one event and only run mediocre times in the others (like all of Wanjiru's other non-marathon races after Beijing). Ever since Wanjiru and then Tadese took the Half Marathon record from Haile, all outdoor distance World Records are held by different people (not counting 1500 and mile, 3000 and 2 mile etc as different events), all except for Bekele's. I said this in another thread a while ago, but someone else really should have the 5000m record over Bekele. The 10000m is his main and favorite event, he has said this, and it just shows that that event is lacking in good runners that he hold both. Ideally, runners would be specialists and each WR would be by a different runner. Coe was an 800m specialist and the fact he had the 1500m WR just showed a lack in that event, as 3:33 was much weaker than 1:41. That got taken care of soon after.