Nope. It is a right, not an obligation. Your employer can't stop you if your want to test your wings. Though this has actually been around in some form for quite a few years.

I started my first company in 2001 by getting 6 months off from Ericsson. I wanted to during that time, Ericsson would have to allow me to come back and continue as an employee. Quit permamently after three months. Now I'm happily running my fifth startup.