[beardface]

Whether or not it will be profitable is yet to be seen, but I am in the process of doing this.

As has already been mentioned, make sure the IP is in your name. My employment contract is one of those generic 'everything you think is ours' kind of affairs. I don't know if it would hold up in court but wanted at least some security. I'm in the UK, so this may not be appropriate to where you are.

After I proved the idea will work but before I started building the product for real, I went to a solicitor (lawyer) and explained my situation. They draw up a Deed of Understanding and Assignment. This is a document that gives scope to the work and states that my employer has no claim to any work inside that scope. My employer (eventually) signed it, as did I, and a witness. It wasn't free, but I feel safer having it.

Even if you're building a product that won't compete with your day job work, do all you can to make sure your employer can't claim it. If you do start making profit and your employer isn't (or is just greedy), it's not outside the realm of possibility that they could find out and claim all rights to it. I also don't see why this couldn't happen after you've left your job, if you built the product while employed.

If you think it's going to go somewhere, speak to a lawyer as soon as you can.