After doing some research, it appears as if many of the MD schools in many states give preference to residents of their own states or states with which they have special agreements. For instance, in a recent UW class, only 10% of the people admitted came from outside the Washington-Idaho-Montana-Wyoming + one other state region.
Which brings up the question: if you have a couple of schools in mind that you'd really like to attend, but live in a different state, how important is to move to that school's home state before applying?
Is the situation the same for DO schools?
it tends to apply more definitively for state-supported schools - that is, the taxpayers support the school to create doctors likely to serve in the state. some DO programs are similarly oriented (e.g. looking to supply Drs for a small rural state) - many DO programs are wide open, and most if not all private (not state supported) schools are open to those out of state.
it takes some doing though to establish residency in a new state in order to apply as a resident - you cannot just move there and then apply 
L
One other thing to consider- at many state schools, you cannot change residency status once you begin, unless you take time off from school. My school even offered people the chance to defer admission, in order to become a state resident. Otherwise, you're stuck with out of state tuition for the entire four years.
I would strongly suggest checking with the school's registrar's office to find out if there are any exceptions for establishing residency. This doesn't generally help when applying, but may help in getting in-state tuition once accepted.
Lisa and Epidoc are correct! Usually “private” schools (MD & DO) do not have
specific residency requirements since they aren’t state funded.
If there is a particular “state supported” school you are interested
in, your best bet may be to contact the school directly regarding
their residency requirements. The process for establishing
residency varies from state to state.
Best wishes in your search! 
Jeff in Seattle
I may be wrong, but I think even some state schools allow you to establish residency in school, Colorado comes to mind. However, that only helps you with tuition and not admission.
Also, If you live in one of those states you mentioned with regards to UW, they have a broader “net” than other states. I believe there is a group of western states that allow you to have residency status within that region. That only helps if you live in CO, WA, NM or some others. Check with the school’s web sites. Sorry I’m being vague.
How flexible are you about moving just for the purposes of admissions? You may have to live in your desired start for at least a year to est. residency, and you may need to have a job while doing it. (taxpayer!)
If you are close to applying, I would say to go ahead, make it clear to the school of choice that you ultimately intend to practice in that state (if true), play up any family ties in the region, and hope for the best. If you don’t get accepted and are dying to go there, then move, apply again, and throw yourself at their mercy!
Good luck!
Theresa
TheresaW, I think what you're referring to in the Western states is the WICHE agreement- but it isn't as straightforward as it might seem. You CAN be considered in-state from out some out of state schools- but not universally. My understanding is that some of the states will agree to make up the tuition difference to the state that accepts their students. Consequently, for budgetary reasons they limit the number of people they will do it for. And, it's not universal. For instance, as a Colorado resident, you can't be considered a resident in the other states, even though some of the other states folks can be considered residents in Colorado.
Also… unless things have changed, I don't believe you can establish residency in Colorado during school unless you can prove you worked and paid taxes during that year, and were totally self-supporting. Out-of-state students at the undergrad institutions in Colorado generally had a very hard time establishing residency there and I don't think UCHSC is any different. I know that in Michigan we can't change residency once we start, except in a few, rare cases.
But I refer you back to your friendly registrars for verified answers! (You can usually find the residency requirements on the Registrar's Page on the school web sites).
In addition to talking with the registrar's office, I would be sure to talk with the med admissions office. Also talk with the financial aid folks as they often are the authority of how much you have to pay, and whether you qualify as a state resident.
Cheers,
Judy
One of the schools that I would love to apply to is in the next state over from where I reside now. However, I was born and raised in that state, so I went through the public school system. If one did not graduate from a high school in that state, (MA), then you need to live in the state for 5 years prior to applying. Since I dropped out of school in the 10th grade, I'm not sure where I stand. With my luck they may tell me to take a hike (in a nice way of course). It looks like a call to the admins office. Bottom line to the orginal poster is that each state is different - but 5 years seems the longest I've seen so far!