Anywhere with a fuss-free homeschooling statute.

As the children grow older, it's a good idea to look for a place with strong advanced curricula for secondary school students, such as UMTYMP

http://www.math.umn.edu/itcep/umtymp/

and PSEO

http://www.ohe.state.mn.us/mPg.cfm?pageID=797

in Minnesota, the PSEO clones Running Start

http://www.k12.wa.us/runningstart/default.aspx

in Washington State, Youth Options

http://dpi.wi.gov/youthoptions/youthop1.html

in Wisconsin, or other similarly named programs in a few other states. Especially cool is to have an explicit high school for the gifted such as Montgomery Blair

http://www.mbhs.edu/

in Maryland or Thomas Jefferson High School for Science and Technology

http://www.tjhsst.edu/

in Virginia or one of the several state "governor's schools" for highly gifted learners. Some families move to Reno, Nevada for the Davidson Academy of Nevada

http://www.davidsonacademy.unr.edu/

but so far I haven't tried it.

It really depends on the part of the country you're in (or interested in moving to). I grew up in North Carolina where lots of families move to raise kids. Growing up, I saw growth of epic proportions. Families would migrate in droves to the triangle area from places like Ohio, Massachusetts, and New Jersey (this is still going on and the whole region is having some pretty serious growth problems). From what I gathered back then, and looking back on it now, I would say that good schools, good climate, nice people, safe area, reasonable cost of living, good colleges in state, and a nice outdoor environment are all paramount. These are mostly the reasons that people move to areas like where I grew up ( Cary, North Carolina).

Near family is my wife's primary deciding factor.

Near a lot of opportunity is my primary deciding factor. I grew up essentially on a farm in a small enough town that we had no (and they still have no) stoplight. I'm very thankful for what I was given, and I take pride in my blue collar education/skills, but I was the only computer geek in my school, and while we were only a few minutes from a city of ~500k, I never built a social circle of people who shared my hobbies. I'd like my children to have the opportunity to experience a lot of cultures and be able to seek out situations that will allow them to excel at whatever skills they have.

Fortunately my wife's family is in a big city, so we lucked out. :)