I was unsure if I wanted to be a SAHM (but was pretty sure I did) and I returned to work for four months before finally resigning. However, part of my decision to return to work was a company policy regarding paid leave (short-term disability) and insurance. My particular company had paid short-term disability for maternity leave and if you gave notice that you would not be returning to work, you were required to pay back that benefit in full. Also, you would be required to pay the company portion of your insurance premium for the time you were on leave (the company paid 75% which was a few hundred dollars a month). So it was to my financial benefit to return to work for at least a week. I had several friends who came back to work after having babies and immediately gave notice and were therefore not required to pay back any leave or insurance. Anyway, some people say that's taking advantage of the company, but I figured I'd earned it after working there for 10 years. So my advice is to make sure there aren't any financial issues you'll have to deal with if you resign now.
If there aren't any financial issues and you are 100% sure you want to be a SAHM, then I think it would be best if you called your employer and just let them know you don't plan on returning. I'm sure they'd rather fill your position now rather than later.
Good luck!