At a risk of being dull, I thought that it had been scientifically determined why toast generally landed butter side down.
First, of all it doesn't. It has nothing to do with being buttered. It just is that the buttered side is generally also the side which is initially 'up'. This gives you your initial spin state. As it falls from the table to the floor it generally rotates an uneven number of times - I can't remember if it is one and a half or a half spin, I have never dropped toast myself; only burnt it. Of course the butter-down incidents are more memorable than the butter-up-and-ate-it ones (although, I would throw it in the bin and start again, not that this has ever happened, although plenty of burnt toast has been thrown out unless I have salvaged it through 'scraping'), so an inherently, statistically less likely event is downplayed by perceptual bias and cultural dissemination makes it into a 'law'.
If the table was lower/higher this wouldn't happen so much.
Cats also don't always fall on their feet, but they make every effort to. So, strapping buttered toast to a cat's back, inverting the whole thing and dropping it from a suitable height will lead to the cat still following its natural instinct and landing on its feet with the 'butter-up' on its back. Provided, of course, that the manner in which the toast has been strapped on does not constrict the cat's ability to twist its spine, as without this it will most likely land on it side, yowl and run off (even so, the toast will not land butter side down).
Incidentally, eating this toast now that it has been in contact with the hair of the cat's back is much the same choice as whether you would eat toast that had landed on the kitchen floor (on which the same cat had sat). I suppose there may be a greater risk of the warm toast attracting fleas into it, if the cat had these, but then all of these extreme possibilities are a tad moot as most people will learn from their mistakes and be less clumsy in future, which is obviously of greater benefit to all aspects of kitchen hygiene, safety and frugality.