Maybe McDonald's won't buy it because they are concerned about liability - there's a large amount of info coming out that Splenda isn't as safe the the producer would like people to think. Do a google search on "side effects of Splenda" and you'll see what I mean. There's another FT thread right now on Splenda intolerance in the Women Travelers forum.
One scientist writing about Splenda described it as being closer to DDT than sugar in structure. Something to think about.