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Results seemed to be the same for both Google and Fast.
My thoughts were along the following lines as I had submitted the domain (per se www.mydomain.com) to the engines, then they saw nothing different between www.domain.com/index.html and www.domain.com/keyword.html (maybe if I submit www.domain.com/keyword.html I may get a change in results)
Anyway I will continue experimenting and will let you know the results.
The problem with your idea is that the SE has no idea what the filename of the default page is unless you submit the full URL including the filename... and if you do that, you aren't referring to it as the default page, so there's no point in changing the default page to something besides index.htm or index.html. Remember, you can always submit [yourdomain.com...] even if it isn't your default.