Some Japanese keyboards do this. Tiny spacebars and extra modifier keys. I've always wanted to buy one and put keys like backspace and delete on my thumbs.
The Kinesis Advantage keyboard effectively does this, except the left half is for backspace. Since it has very conveniently placed modifier keys like ctrl under my thumbs, I remapped capslock to shift so that it's on the home-row.
I think the left side is more suited for backspace. Which is how my Compaq keyboard is setup - the key is physically divided into two.
Perhaps the spacebar is just too big, and could be divided better into other modifiers. My left thumb is usually wasted (though not on a Mac, where the thumb can easily reach the Command key.)
Some people however - seem to alternate thumbs on the spacebar - or rather use the other hands thumb to the hand that just typed a character. I personally always use the right thumb to space.
