"I believe 48.3 was the pacemaker's time, not Kipketer's. From what I remember, Wilson's splits in all his 1:41 races were pretty sensible - not inside 49 anyway."
Well, for whatever it's worth, I just looked this up in the IAAF WR Progression book, and they say that the pacemaker at Zurich (Tengelei)hit the 400 in 48.10 (I was there and didn't recall it as being quite THAT fast), with Kipketer's official 400 splits as 48.3 and 52.9. I would presume this source would be correct. I guess that if you can run 1:41, a 48+ first lap is at least somewhat "sensible."