Interesting idea, I wonder how it will turn out in a year. I think the initial motivational boost will drop after a while and they will have hard time keeping their sales people motivated after that.
Other types of employees find the source of their motivation in other things; some of us programmers are motivated by creating cool stuff, teachers are motivated by seeing the kids grow and becoming more educated, these are mostly internal motivations as opposed to external motivations such as commissions and bonuses. Maybe it's just because I don't really know the work of salespeople very well, but I can't really imagine them finding an internal source of motivation in their line of work.
The only thing I can possibly think of that might become a source of motivation for them is helping clients finding the right solution.
Why is it hard to imagine that selling the 'cool stuff' developers/engineers make (and being the best at it) is motivation enough? Without sales people all the cool stuff we hackers create, would be seen and used by almost no one.
The art of selling, even without the ugly side of it (which there's plenty) is pretty amazing. We all admire guys like Steve Jobs but don't realize that he's the best salesman there is.
Don't forget the satisfaction of helping people! When your widget fills a need, not merely a want, in a company or team, you can feel good at having met that need.
There will always be the slimy, rip-his-granny-off kind of salesman because people are like that. However, the flat-pay system would reward the good kind of salesman more often than the bad kind, and would bolster friendly competition within the company.
Don't overlook the large amount of social interaction sales people get, for a people person that is a huge perk and likely just as satisfying as building something cool is to a programmer.
If they feel they aren't being compensated properly for their performance then yes. In other jobs the 'stars' will tend to find work elsewhere unless they are suitably recognised.