As a prospective employee wouldn't you like to kick the tires before making the leap into a new job that you might find yourself wanting out of six weeks in? There's so many things you just can't know about a company until you're on the inside.
Edit: Remember that 37s has a lot to offer: Remote work with great people, top-tier name recognition in the tech world, and a long history of building and releasing open source tools. That's a really compelling package compared to some of the bigger names in tech.
No matter how you look at it, you're asking me to take on significantly more risk with this process.
For developers that are in demand, it's a tough sell.
You can't really expect someone to quit their existing job just for a chance to work with you.
If I'm job hunting, I've got other offers on the table. Ones that do guarantee a salary for more than a month. Ones that include benefits. Ones that expire long before your trial hire would be over.
If you try using this method, I don't know of a top notch programmer that you would be able to attract, simply because the hiring process is too painful.