Anecdotal evidence, but my experience...
At 6'5", over 200 pounds, and an ex-track rider, I can attest personally to the advantage in front end stability (and therefore steering) with a 1 1/8 inch top tube versus a 1 inch tube. I had two track frames built back to back (a long story) and the second had the oversized top tube. All other tubes were the same. The only other variable was the need to build the OS frame as lugless, instead of the cast lugs on the smaller tube.
The larger tube was significantly more stable on the banking making the bike more manuverable when riding in the pack. I have since added OS TT road bikes to my stable as well (over the 10 limit - Shhhh...), and I have found much the same story with them. There is much less "wag" in the top end of the bike. For data purposes, all these frames are 61cm or larger and steel tubing.
I strongly believe that 1 1/8 inch top tubes are a big advantage for a larger frame but probably worthless on a more normal sized bike.
My .02 worth.
Mark Ritz
Arcata, CA
http://www.kinetic-koffee.com