California at least has quite reasonable laws on this stuff. As long as you're not doing anything shifty and you are strictly using own resources and time there's not much to worry about.
Whether you should talk to your employer mostly depends on the company you work for and the laws in your area.
it's better to ask for forgiveness than for permission!
on a more serious note, (i'm not a lawyer). I would look to see if you're competing with your employer, if you are not than there's not much to worry about, if you think you might be, check with an attorney.
Well you don't have to be competing directly. If you work as a software developer, you could be working on a product thats really different from your employers but its still software that you are developing in your free time.
Whether you should talk to your employer mostly depends on the company you work for and the laws in your area.