3 simple Q's (Who are buying homes? Why are suburbs Rep? Why are cities Dem?)

[DanielX]

1) What are the population factors behind the growth of the suburbs?

Many. One possibility as to why there aren't huge net migrations to the suburbs: there's a population exchange going on. Many parents with children, especially middle-income ones (used to be mainly white ones, but nowadays that isn't neccesarily true), leave the cities for the suburbs. However, the cities compensate with influxes of newly arrived immigrants, young singles, childless couples, homosexuals, etc. who are less concerned about crime, education, large houses, 'family values', etc. 

2) Why are the suburbs so Republican? Is it class, race, family structure, or something else?

All of them. Suburbs tend to be middle to upper-middle income, prime ground for Republicans. They tend more white then cities, and the non-whites tend to be wealthier and more established. Also, parents with children tend to be more Republican. Throw in religion too - suburbs have more practicing Christians and fewer Muslims, Buddhists, Atheists, Wicca, etc. then the norm (although this isn't an absolute rule).

This isn't universal. There are regional variations - compare New England suburbs (which are chiefly center-left) with Southern/Midwestern suburbs (which tend to more to the right).
Also, unlike most ethnic groups, there is little variation among blacks - with very few exceptions, they tend strongly Democrat regardless of income or location (although rural areas are very slightly more Republican, mainly due to social conservativism).

3) Why are the cities so Democratic? Is it class, race, family structure, or something else?

See above.