Moved in with Mom, still have house in another state. Do I have to change state residency at some point to stay legal?

You do not have to change your legal residence if you help your Mom...even if you live with her for awhile. I am not sure what you mean by staying "legal." If you are getting gov. benefits from another state, they may have some residency requirements. If you take a job in your Mom's state, you may have to file a seperate tax return. If you are renting your home and plan on moving in with Mom, you would have to get a new driver's license and registration. That is all I can think of that would be "legal."

Thanks for your input...have a house in Ohio...moved in with Mom in Michigan 7 months ago...have no job...she supports me while am living with her...was wondering if I have to change to Michigan to be legal with registration for car and driver's license and taxes for 2011.

I am not an attorney or accountant, I have just moved a lot :o) From my experience, you can have a legal residence in one state and work/live in another. Any income you make in your any state needs to declared and filed in that state. Of course, you will continue to pay taxes on your house in Ohio. (are you renting it? That will have to be declared in Ohio, too.) You would file a "partial year resident" tax form in each state.
As far as car issues, if you are planning on staying in MI, I would check your local DMV on how long before you have to get a new license and plates. You will also have to change the title. To save a little bit (and if the expiration date is close), you can wait until your auto's registration date is up then change over.