Your on dodgy grounds right now, as the idea was developed in the course of doing business for your employer. Any IP rights will probably belong to your employer, you either need to leave before you work on it or negotiate an agreement with your employer.

The later might not be impossible but depends on your company, Salesforce actually started in a similar way with Benioff leaving Oracle to pursue his vision, Larry Ellison actually not only gave his blessing but also invested in them. The investment bank I was working for we're completely happy for me to work on a non-competing startup.

Salesforce actually have an investment fund especially for startups building products on Force.com, have you considered applying for it as an alternative to your day job ?